[Warning: The following contains spoilers from the series finale of Pretty Little Liars! Read at your own risk!]

At the end of Pretty Little Liars, A.D. was one of their own — sort of.

I. Marlene King pulled a fast one on fans by revealing in the series finale that the girls' most deadly stalker was the twin sister of Spencer (Troian Bellisario) — Alex Drake. Once the story was told through Alex's eyes, fans could start to piece together the small clues that had been delivered throughout Season 7 to see how multiple times we thought we were watching Spencer, we were actually watching her evil twin.

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On top of wrapping up the show's central mystery, the two-hour series finale of Pretty Little Liars also delivered a happy ending for each member of the core cast — though some were more twisted than others — and dangled a few threads to welcome fans back to the world of Pretty Little Liars later on. TVGuide.com talked to King about finally deploying the twin twist and how she'd want to return to Rosewood in the future.

Sasha Pieterse, Lucy Hale, Ashley Benson, Shay Mitchell and Troian Bellisario, <em>Pretty Little Liars</em>Sasha Pieterse, Lucy Hale, Ashley Benson, Shay Mitchell and Troian Bellisario, Pretty Little Liars

How long ago did you decide that you wanted Troian Bellisario to play A.D.?

I. Marelene King: I would say it was when we took our hiatus between Season 5 and 6. I really had to put my nose to the grindstone. I came up with three ideas and then we pitched them out in the room and we all agreed that the one we were most excited about was Troian as A.D.

Who came in a close second to Troian?

King: Wren came in a close second. Wren, Mona and Melissa were all sort of in play. I think the way we used Mona was more fun than having her be on the team for bad, malicious reasons. It really came down to who we knew we would have. What actors did we know would be available? We knew we had to use one of our series regulars.

For Charlotte's A reveal you were pretty adamant about not doing a twin twist. What felt right about doing that throwback to the books for the series finale?

King: I've always been fascinated with dolls and twins, I guess. The room was fascinated with dolls and twins. It became sort of our own obsession. It was something we couldn't stop thinking about so it felt like the right time.

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How long after deciding that it was going to be Troian did you tell her she was going to be A.D.?

King: Right away. It was before we even came back. We went to dinner or something while we were still out between seasons 5 and 6.

Did you tell her to keep it a secret from the others?

King: I was going to tell the PLLs but Troian specifically asked me not to because she didn't want them to know when she was A.D. in a scene. I think that was really smart.

Let's talk about Alex's accent. Was that a creative choice you made or something that Troian wanted to try?

King: I knew that she was going to have a connection to Wren and Wren is British. Troian specifically wanted it to be not Wren's accent. She wanted it to be less upper crust and someone who was raised more on the streets. We kind of worked together on that part of the mythology of Alex.

You left a couple of hanging threads at the end of this finale. If Freeform called you tomorrow and said they want more, would you lean more towards a Mona spin-off or Pretty Little Liars: The Next Generation?

King: It's a tie for me! I love them both. I love the world and I love the characters. If Freeform called tomorrow I'd love to play in both worlds.

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This was sort of a strange happy ending for Mona but at the same time felt very fitting for her. Why did you guys want to go in this direction?

King: Mona has been through so much. She made the sacrifice because she begged Hanna, "Please don't make me play the game," and Hanna is like, "Come on, you can win!" Mona should have her happy ending. She's living in France. She's always loved her French music and French literature. She's living in France with her French hottie in a doll shop with her real life dolls in the basement. I wanted her to have what Mona's version of a happy ending to be.

You managed to get so many people back for the final season, but was there anyone who couldn't make it?

King: We really wanted to get [Drew van Acker] back, but he was on Training Day. We wanted to get Cody [Christian] back to play Mike to be at the wedding but we couldn't get him either. Those were the two that got away.

One of my favorite moments from the finale was the tease of how the moms got out of the basement. Have you have guys ever teased explaining how they escaped or did you purposefully want to leave it a mystery?

King: We pitched on things that made us laugh but nothing made a lot of sense, so leaving it up to the fans' imaginations is the most fun way to go, knowing that whatever happened was dark enough that Pam didn't drink for a year after that.

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Did the girls weigh in on what they wanted their happy endings to be at all?

King: Ashley always wanted Hanna to have a baby so having Hanna get pregnant made her happy. Shay and Sasha really wanted Emison to find a way back to each other, so I think that made them happy. I don't know if Aria wanted to live happily ever after. I think she wanted to be blown up or something like that but I couldn't let that happen.

Is there anything that you would like to say as your final words to fans at the end of this crazy journey?

King: Just thank you. We are just so grateful our fans. We wouldn't have made it seven years. We wouldn't have made it seven years without their loyalty and passion. Even when they love to hate us, they still are passionate. I'm just really grateful to everyone. We'll always still be here. We're not going away. We'll still be our worldwide family and we'll keep talking, hopefully about new shows.