SPOILER ALERTDo not read if you have not watched all of “Pretty Little Liars: Original Sin,” now streaming on HBO Max.

The “Pretty Little Liars: Original Sin” mystery is of “A” is officially solved. During “Final Girls,” the tenth episode of Season 1, the liars, played by Bailee Madison, Chandler Kinney, Zaria, Malia Pyles, and Maia Reficco, came face-to-face with A, the man who had been terrorizing them and their mothers. Plus, they found out that not one, but two men were behind the slew of murders in town.

Here, co-showrunners Roberto Aguirre-Sacasa and Lindsay Calhoon break down the major reveals, when they decided who A would be, what the cliffhanger means for a potential second season — and how those “Pretty Little Liars” and “Riverdale” Easter Eggs came about.

I want to start out with the theme of twins that runs throughout; it starts with Karen and Kelly (Mallory Bechtel) and ends with Angela (Gabriella Pizzolo) and Archie. Of course, that was also a big theme through both of your past projects. What it is the draw there?

Roberto Aguirre-Sacasa: That’s such a good question. It’s so embedded in the horror genre; it’s such a trope. I will say that we did not know at the beginning of the season that “A” was going to be Angela’s brother. That that emerged later. It’s funny, I didn’t even really think of them as twins, though I guess they were twins!

Lindsay Calhoon: It is something I think I’ve forgotten about! When we really dug into it — I think about mid-season when we sort of landed on who “A” was, who his father was and the Principal Clanton [Robert Stanton] of it all, we did sort of create this timeline of when Rose [Jeanette Bonner] and Principal Clanton met, when she became pregnant with twins. Now that you mention it, there is sort of an uncanny parallel between Kelly and Karen’s story and Archie and Angela’s story and part of that is this idea of one twin lives in the shadow.

Aguirre-Sacasa: I’m surprised we didn’t get a note from the executives, saying, “There can’t be another set of twins on this show,” because it is like, how many twins are there in Millwood?

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Robert Stanton in “Pretty Little Liars: Original Sin.” HBO Max

You said you didn’t know Archie would be “A” when you started. You started writing this as a mysterious villain and you didn’t know who he was?

Aguirre-Sacasa: I felt like when we pitched the show, we presented a list of 10 suspects to HBO Max, but we didn’t tell them who it was. We were like, “You have to buy the pitch. We’re not going to just tell you the solution.” And weirdly, even after they bought it, I don’t think they asked. We were asking, is it Angela’s mother? Is it Sheriff Beasley? And then we were breaking episode three, where you see that “A” is under the bleachers while Sheriff Beasley [Eric Johnson] is on the stage. So, there were various versions in play. It was about halfway through the writers’ room that we started landing on specific things, like the idea of two people working together in tandem as “A.”

Who else was in the running to be “A”?

Aguirre-Sacasa: Mrs. Waters for sure. I also feel like we talked about Angela’s father being “A,” not knowing that Angela’s father was going to be Principal Clanton. There were a lot of people on the table.

Calhoon: This was early, early days and there was a long list of suspects. I think we knew that it had to be somebody who was connected to the school and present in the school. We were sort of obsessed with Mrs. Murray [Barbara Tirrell], the librarian, and we thought, “Oh my gosh, Mrs. Murray might have them within earshot in detention! She heard the girls say, ‘I want to kill Karen Beasley. Mrs. Murray could be pulling the strings. Is there some big reveal that Mrs. Murray is Rose Waters?” We definitely talked about Mr. Beasley. We talked about Wes [Derek Klena]. I think even at one point early on, it was discussed whether it could one of the PLLs be “A”? We were were so open to every possibility of unmasking there. I love that this was so a story about mothers and daughters, and at the end of the line, they’re facing off with a father and son. I thought that was just a really interesting way to sort of bring everything together and have it all dovetail.

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Chandler Kinney and Bailee Madison in “Pretty Little Liars: Original Sin.” HBO Max

Three characters in Season 1 of this show were sexually assaulted. Can you talk about that choice, and whether you thought that may be a lot for the audience to handle? That’s a very heavy storyline for one character to go through, let alone three.

Calhoon: We talked about this very early on with each other. We we had a conversation with RAINN, who was partnered with us with resources, which is really important to us. One of the themes early on in developing the show was we knew we wanted this to be a slasher horror show, but we also wanted this to be a grounded horror story and to really [discuss] how can we talk about what some of the horror stories are for young women that are real? I think it’s hard to not go to sexual assault, not to go to rape in that story, because it’s such a prevalent real-life horror.

Having three characters you know — it was interesting to us, because I think there was a subconscious that I wanted to tell that this is real, this is prevalent, this could be someone you know, it can be a stranger. In the case of Tabby and Imogen, this became an unfortunate bonding story between them that supported their sisterhood. This idea that Tabby was on this path of who she thought is the correct or the right type of assailant: “I’m going to look at jocks; I’m going to look at Tyler who I imagined to be the embodiment of toxic masculinity.” And the real of that and the statistic of that, is it can be someone you know, it could be someone you’re not thinking about. So we wanted to tell that story as sensitively as possible.

And we wanted to protect our actresses, and made sure that when we shot these scenes that they were sensitive, and this was something we never had to show but could really tell this story through the girls opening up to each other. Sydney’s [Sharon Leal] big sin in the show was Angela opened up to her and told her about her assault, and rather than the girls helping her and listening to her, they shunned her — and what a horrible thing for anyone to do. For us, we really wanted to see the parallel and the difference between the way a group of friends handled this story 20 years ago and the way a group of friends is handling the story in the present. When Imogen and Tabby tell their story to their girlfriends, they are there for them, they support them. It was never about being salacious. It was always about telling these stories in a meaningful way, and having two time periods and two groups of young women we were following that allowed us to see the differences and the parallels between two stories of assault in a horror show.

Obviously this show includes many nods to the “Pretty Little Liars” world — especially with the roadtrip to Rosewood. In that episode, we got the “Sisters of Quiet Mercy” mention too, so was there a possibility that could have been a roadtrip to Riverdale?

Aguirre-Sacasa: No, it needed to be a road trip to Rosewood. If we were going to leave our town, I think it needed to be wedded in the “PLL” universe. Never say never, but I think the idea was if we’re going anywhere, we had to go back to that OG town.

Yeah, for sure. But you also went with the name Archie for the big bad. I assume, Roberto, that was you!

Aguirre-Sacasa: No! For months and months and months, it was Andrew Waters. When you go through the process of doing shows, you have to clear names. I think it was like three days before we shot, and we got a note back that Andrew didn’t clear. So, we just were submitting a dozen “A” names and none of them cleared but Archie!

Calhoon: This was so not Roberto. I think we were joking around in text with our finale director, Lisa Soper, and we were like, “We’re having the hardest time! We have the big reveal. ‘A’ is a big thing for us, we need an A first name.” Lisa joked, “Are we trying Archie? Is it going to end up begin Archie?” And I was like, “You know what, I’m sending it straight to the clearance department.”

Along with the reveal of “A,” we also found out that Aria and Ezra are adopting the baby. Was that the plan all along?

Aguirre-Sacasa: No, that was late in the game as well. It was even a question of, is Imogen giving the baby up for adoption? That was something that we talked about a lot. It felt like Bailee had done such a heroic job of being pregnant the entire season that, in success, if there were a Season 2, we wouldn’t necessarily want Bailee babysitting a baby while all the other girls have fun going out and clubbing and things like that. It felt like the right choice for that character, so it was late in the season.

And then one of our writers and “PLL” super fan pointed out that not in the show canonically but in the social media around the show, Aria and Ezra, who had been struggling to have a baby, had adopted a baby. And so it docked perfectly with Imogen’s story.

Calhoon: That writer, Katie Avery, she’s such a “PLL” super fan. She’d read every book, she’d seen the show as many times as we did, and I loved that — and I wasn’t as well-versed in the deep social media canon following the show. So I loved that we could dock with the canon of the original show and those characters. It felt like a really beautiful end of that story.

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Lucy Hale and Ian Harding as Aria and Ezra on “Pretty Little Liars.” ©ABC Family/Courtesy Everett Collection / Everett Collection

Does this mean going forward, they could pop up? And would that mean Lucy Hale is Aria or Katy Keene?

Aguirre-Sacasa: She’d be Aria, for sure.

Calhoon: We’d make Lucy Hale do the twin work!

You guys do love the twin work. So with this planning, some obviously came much later. But the end shows who Archie is and that he survives. Should you get a Season 2, will it be the same big bad — even though that would take the mystery away?

Aguirre-Sacasa: That’s a really good question. We’ve been talking about it, and brainstorming it. I don’t think it necessarily means that “A”/ Archie would be the villain. But he’s out there. We wouldn’t necessarily be following his exploits as a serial killer or as a slasher villain.

Was there ever a discussion to show his face?

Aguirre-Sacasa: There was a discussion for sure. We got a note about it and we felt like, you don’t really see these guys’ faces; you don’t really see Michael without his mask. You don’t really see Jason without his mask. You see them in shadow. It came up a couple of times, actually.

Knowing both of your backgrounds, I have to ask whether you plan to keep this series a straight horror/drama or whether you’d bring any supernatural elements into this.

Aguirre-Sacasa: I say no supernatural elements, but I said that about “Riverdale!”

Yeah, you did. That’s what I was going to say.

Aguirre-Sacasa:  I did say that for four years on “Riverdale.”

Calhoon: I agree. Never say never, as Roberto says. We love that this is a show in the slasher genre, so I think we want to continue with this being a slasher show. We flirted with the supernatural and subverted it, I think, in a really lovely way in Episode 4, watching the tweets as Imogen says, “I’m going to contact my mother.” There was a lot of, “Oh, here we go. Ghosts are coming in!” I loved that that scene was not a supernatural scene, but was a cathartic scene and was a scene about releasing grief and Imogen processing. So there’s always fun ways to flirt with other other sub-genres of horror while keeping it in the slasher genre.

Aguirre-Sacasa: Who knows? Maybe there will be a spinoff of the spinoff “Ravenswood.”

This interview has been edited and condensed.