SPOILER WARNING: This story includes major plot details for the Marvel Studios limited series "Agatha All Along," currently streaming on Disney+.

When Jac Schaeffer signs onto Zoom roughly 18 hours after the two-episode finale of her series "Agatha All Along" debuted on Disney+, she looks at once relieved and slightly queasy.

"From when it dropped to about an hour ago, I've been feeling pretty shaky," the showrunner says with a laugh. "We dropped a lot of content -- a lot of moments."

Schaeffer isn't exaggerating. The episodes, titled "Follow Me My Friend / To Glory at the End" and "Maiden Mother Crone," are chockablock with surprise twists and revelations that upend the story of Agatha Harkness (Kathryn Hahn), Billy Maximoff (Joe Locke) and the coven of women who venture with them onto the fabled Witches' Road. For one thing, it turns out the Witches' Road never really existed -- until Billy unknowingly summoned it into being. For another, "The Ballad of the Witches' Road," the song that supposedly outlines the trials of the road, was really created by Agatha's young son Nicholas "Nicky" Scratch (Abel Lysenko) as a way to pass the time while they walked along a country road together in 1750s New England.

As was the case with Schaeffer's previous series, "WandaVision," "Agatha All Along" grounds these magical plot turns in the characters' emotional storylines: Billy creates the road as a way for him to find his long lost brother Tommy; after Lady Death (Aubrey Plaza), a.k.a. Rio Vidal, takes Nicky into the afterlife one night, Agatha wields the "Ballad" to con unwitting witches into letting her drain all of their power -- and as a way to remain connected to her son.

Schaeffer's deep investment in the care and craft of this kind of storytelling is unmistakable, as is the satisfaction she gets from finally being able to talk about it all without worrying about spoilers. In a wide ranging conversation with Variety, Schaeffer shared how she and her writing team first conjured these storylines, why she wasn't sure she wanted the road to be created by Billy, what the backstory was for Agatha and Rio's romantic relationship, why we never learn who fathered Nicky, why Tommy isn't in the show, what she regrets didn't make the final cut of the show and the post-credits tag she wrote for "WandaVision" that never got used.

It was about a month into the writers' room. We had been working diligently on the construction of the road, putting a lot of love and attention into what this would be like. We had this idea that all the trees were the witches who had died on the road. We were taking it really seriously. Then I had this idea that maybe Billy made the road, and Agatha puppeteers him to do it. Strangely, Mary Livanos, our executive producer, had the idea of Billy making the road at the same time.

We had already been circling this idea that Agatha had walked the Witches' Road with her son, that he was perhaps buried on the road. But then Giovanna Sarquis' had this idea of Agatha singing the "Ballad" on the road with Nicky, and it all started to slot into place. Billy made the road, Agatha did in fact walk a road, but it was a real road in nature with her son, and he's the one who came up with the "Ballad." She took the "Ballad" and turned it into a con.

I am obsessed with "The Usual Suspects," and it felt like Agatha being Keyser Söze, with Billy having with the Kobayashi moment where all the pieces come together. All along the way, even this week, I was feeling conflicted about Billy having made the road, in terms of it being a duplication of "WandaVision." But I could never resist the truth of Nicky coming up with the "Ballad" and that being the heart of it all. It was just irresistible.

I have had the pleasure of working with these characters in the MCU, who have such power as to destroy stakes, to melt a narrative. You have to be really careful with that. If Wanda and Billy can do anything they want, everything falls apart. I think that's the hesitation. I feel we did it with intelligence and fortitude. We decided that everything in his room had connections to the road and that, if you're watching the [end] credits, we're telling you that from the very beginning. In fact, if you watch the credits of "Agnes of Westview," there are single frames of the witches that Agatha kills. So we're saying everything upfront, it just doesn't knit together until the end.

But I also felt the overwhelming desire to dip back into the "WandaVision" playground of pop culture illusions, and this felt like a really authentic way to do it, that William Kaplan/Billy Maximoff loves magic, loves witches, and loves horror, and that that can be the fabric of the show, much like Wanda and her obsession with sitcoms. So it ended up feeling very right.

I think, too, that Wanda made her sitcom reality out of grief and desperation, and I think Billy made a lot of his out of love for the witch community, and that is really lovely and beautiful. Even though "Agatha All Along" is a darker show, and there are horror elements to it, there's an enthusiasm and an eagerness and a joy to it, that I think is born of his psychology.

I will say that we played with her hoping that it was Nicky. There are moments, even with the evidence right in her face, when she lets herself believe a little bit that it is her son. And Rio brings her back to reality.

It was less about her performance than what we chose to use in post. We captured more things hinting at the truth than we actually have in the show. She gave degrees. In Episode 2, when they finally make it to the road, I think I did three or four takes of that single shot pulling Agatha out of the strangler fig tree onto the road. I think I did one where she barely looks at Billy and then I did one where she really gives him the side eye. Kathryn fully understood that we needed to get variations in order to calibrate it in post. In our early cuts, there wasn't enough. There were some Marvel notes about tipping our hand a little bit more, so that the breadcrumbs were there. I agree with that. I think it's calibrated well in our final cuts.

We kept the camera off of the leaves. I thought we weren't going to see the green witch figurine, but you do post on a tiny little screen, and I didn't realize it actually is right there in the back. So it revealed a little bit more than I was anticipating, having stared at it so long. But I don't know, I think it actually worked out that people were starting to piece it together. You need certain reveals to either consciously or subconsciously feel inevitable for the audience in order to earn the ones that are completely off the spine, and aren't predictable. Like, you can't predict the Nicky stuff. You can't just launch an audience into a reveal that has no basis in what's come before.

So I think it's OK that people were formulating that, because all of the theories that I saw were: "Billy has made a version of the road." No one was saying the road doesn't exist. That's the real reveal.

That was very intentional. That's Mary Livanos looking out for the larger tapestry of this family and the visual language, and Wanda and Billy having this thorny relationship with the larger public. I think that's really integral to the Maximoff experience.

I don't remember who came up with that. I wish I could. But we all loved it, because there's a sense with Agatha that nothing is sacred, and that she is so willing to participate in witch-on-witch crime. So that felt really right. But seeing Agatha soften because of Jen's power is really one of my favorite moments.

Well, initially, when we wrote the character of Jen -- when we wrote all the characters -- we had conversations about race, but we didn't land emphatically anywhere [on what race the characters would be]. Sarah Finn, our casting director saw a lot of different performers for the roles.

When we hired Sasheer, she was very interested in research. In her comedy set, she already had a bit about witches, that was based in her research. We liked this idea that Jen was a root worker and a midwife, and we actually put Sasheer together with this professor that we found to do a deeper dive. Everything kind of grew from there. I'm so grateful for the integrity that she brought to that process.

No, it just became more powerful. We always knew that Jen would make it to that trial, that Agatha had bound her, and that she would get herself out. But it just became more resonant when it was Sasheer standing her ground and surviving. It wasn't a conversation about race, it was a conversation about Jen, and what this moment means for her.

We did. We talked a lot about their meet cute. We had these really pretty visuals of Agatha killing people and then seeing Rio across the bodies, like that line, "They met over corpses." I mean, I saw it in my mind. It was really beautiful and also quite funny. And then the room took it in a direction that then they lived together in a cottage, and we talked at length about it, to the point of, "Is Rio Nicky's father?" We went down those paths, and they were very gratifying to explore. I think there's more story there, but we were already biting off so much with the Nicky sequence that was vital to this story, that it didn't feel like we could flash all the way back to Rio and Agatha before Nicky and then get into Nicky. It sort of threatened to tip the boat over, but I think all those discussions informed what you do see of their relationship.

That's a story for another day, but ultimately for me, it's irrelevant in this story.

The truth is that even if I did have an answer, I don't cover it in this property. So it is fodder for a story for another day, that I maybe don't have control over.

I came up with it, and [my co-writer] Laura Donney loved the line so much, she was like, "I feel like I could have written this," but she said it in this lovely way of, "it is born of our work." It's my favorite line in the show, because I love the ambiguity of it. I welcome the interpretations.

For me, I believe it's about the character of Agatha making something. Her power set is only about death and destruction. She can't make anything. She doesn't have chaos magic, like Billy and Wanda, and this is something that she made herself. It required no lying and no conning, and there's something for her that is pure and beautiful about creation.

Yeah. I will say, the reality is, the finale pieces of these projects are very, very hard, because there's a lot of Marvel burden to deal with. There's a lot of fan expectation, there's a lot of familiarity. I deal in tropes of other pop culture stuff, but then, you got to get into the tropes of the MCU. So we're hurtling toward a moment of a crucible, and what everyone's expecting is a redemption arc for Agatha and she sacrifices herself. From the beginning, you feel like that's where it's going, and I was interested in doing that in a recognizably Marvel way, but have it be different, in that it's a kiss, not a blast.

That moment of the telepathy between the two of them was not scripted initially. It's something that we created. It was Megan McDonald's idea on set, and we didn't know exactly what he was going to say. It actually was a late stage idea in post that he would say, "Is this how Nicky died?" I think ultimately it's Billy manipulating Agatha, after being so manipulated by her. He's truthful, but also, he's like, "I have one shot at saying something to this woman, to make her stop," and that's what he chooses. That is shrewd. That's devious, even though it's not coming from a dark place within him.

We knew she would be a ghost from the very, very beginning of developing the show. It was always that we were going to end her as a ghost. She's not in the comics really very much at all, and when she is in the comics, she's often a ghost. So it was like, "This is where we're going." I think when she is her best, she is a mentor. That's what she's meant to be. So the idea of her as a spirit guide for Billy seemed to be a lovely conclusion to this chapter of her life, while also launching us in a new direction.

We talked about it. Were things get hard with ending Marvel shows is like, what's happening next? Do we have that plan yet? Do we have that casting yet? It gets kind of tricky there. So this was as much of Tommy as we could do for the purposes of this show.

That's right.

We wanted to hint at, "This is kind of a rush job, that he finds the closest best body in this moment, and it's not a good place." That sequence was written by Peter Cameron. There's a lot of story economy in setting up the promise of Tommy, who we will hopefully see in a later property. We had to keep it simple. We know that he needs to be about Billy's age, and it needs to be a death of a young person that is tragic, and we want it to be visual. We thought the drowning in the water would be cinematic, but also afford us the opportunity to obscure him.

In my heart, no. I hope this doesn't upset anyone, but Mephisto has never interested me. But we have an early, early document that included him in a potential antagonist, but I don't remember ever really taking that seriously. And also, within the MCU, logistically, it wasn't right for our show. I will say, it was not my intention to toy with anyone with the Mephisto stuff. I think I just continue to underestimate the appetite for Mephisto among the fan base.

No, I haven't had any direct conversations, so I don't know where they are with that. I mean, I wish them well, and I am so excited to see it. I love Paul Bettany, and I love him as Vision.

I don't actually know if that is the plan.

I was really interested in the idea of Agatha's source pain being something very human, very intimate, and very private. We talked a lot about Agatha, the performer and the liar, and I was very interested in peeling back all of the layers, and then just having the story be, Agatha had a son who did not have power, she loved him and he died.

It's so tragic to me, it takes my breath away. It seemed very daring to do that, that the truth behind everything isn't some Faustian bargain with Mephisto and Rio, or that Agatha didn't trade him or kill him. It just was the awfulness of something that people on this planet have to deal with every day. It felt far more powerful.

We had this thing of Agatha looking at the camera throughout the show, and we ended up cutting almost all of them out. The idea behind it was we wanted the Nicky sequence to be the only thing that Billy didn't see, and that Rio doesn't even know the fullness of -- it was Agatha not performing for the first time, but that the audience is let in on it. And in the end of Episode 9, when she says, "The 'Ballad' didn't mean anything, it never did," there's a lovely moment that Kathryn gave where she looks to the camera to convey that she's lying, that the "Ballad" means everything to her, but for reasons Billy could never understand or imagine. We cut it, because it didn't work in the larger catharsis of the later moment of [Agatha saying], "Because I just can't face him." But it's one of my few regrets. I find it so heartbreaking that even then, she pretends like the "Ballad" didn't mean anything -- but the "Ballad" is her kid.

I think so. There are people in my life with circumstances like that. [Pause.] The MCU world is so enormous, and I feel like with this episode, I was taking advantage of the enormity to bring it back down to a human place, to go against the expectation, and hopefully connect with people through that authenticity.

Oh, absolutely. They knew it was going to be the "Mare of Easttown" version, the Sacred Chant version, the Lorna version performed by the cast, the actual Lorna version, the Nicky version, both a capella and in the tavern, and then the Agatha-through-time version. Abel, our actor, when he starts humming it, we needed a proto-melody, so that it would make sense that he's finding it. They even wrote that for us.

We had so many versions of the lyrics themselves. I don't know how much they iterated the melody, because I don't understand that process, but the lyrics shifted. Also, we had a longer Nicky sequence, where we saw scenes that motivated each of the lyrics, and we cut a lot of that. This is something I learned on "WandaVision." Sometimes, you discover that you're actually just patting yourself on the back as a writer, when you're like, "Look at how all these things line up perfectly." You can leave things to the audience to piece together themselves.

So we had, "Tame your fears, a door appears." There was a part of the Nicky sequence where he says, "I'm hungry," that's still in the show. But at the end of that scene, she gives him a knife, and he kills the goat offscreen, and then they roast it over a spit, and she sings to him, "Tame your fears." Then she holds the meat out, and he opens his mouth, and she says, "A door appears." It was super sweet and cute, but it was too much. We didn't need all of that. As we really got into it, it became an act of surrender. It is about the magical quality of music washing over you, and making sense.

That's a Marvel decision. I know nothing more than that.

Yes, I wrote a number of tags, because you always do on every Marvel everything. I love writing tags. I think some of my best writing is in the tags that were never made. I should have a little binder of my tags. They're so fun to write, because you're writing the promise without having to deliver on anything. They're the best. But there are so many things that factor into those. And I was told that we weren't going to do a tag on this show. That doesn't affect my work, or my vision for the show.

Yes. I'll tell you this one, because it's been a long time, and so maybe the statute of limitations is up on it. Initially, Doctor Strange was going to appear in the tag for "WandaVision." It's Wanda sitting on the porch of that cabin, and she's rocking peacefully. And you know how Strange can do those circles around someone, and make them go somewhere? The circle starts around her, like she's going to be teleported somewhere, and she stops it, so Strange has to show up in person. I just loved that so much, that Wanda would be like, "No, I'm not going to go where you want to teleport me. You're going to have to come to my door." It was a good one, but another tag took its place.

I know nothing of a Wanda movie, that's what I can tell you.

No, not in the show, no. The discussion during the rollout is complicated. If I felt it served the watch [of the show] to say, "Wanda's not in the show," I would have just said that. But it's my feeling that it then becomes the conversation. I'm not interested in stringing anybody along. If you look at my comments, they were pretty clear that she wasn't going to show up. But the sort of hardlining it, I think, is no fun. People want to have their theories. I don't want to spoil anybody's party. I just want to gently curate it.

I don't have something lined up right at this moment. I'm trying to figure that out.

I don't know. What I can say is, I love these characters so much, and more to the point, I love the actors and the team behind these shows. I don't know what the future holds, but I have so much love in my heart for this world.