Only in Rosewood could a long-lost evil twin with a British accent stroll into town to reveal herself as the omnipresent stalker Uber A and be met with a resounding, "Yeah, sounds about right." After all, this is a place where it rarely snows (in Pennsylvania), where you don't need a permit to build a massive subterranean doll house, where doctors will artificially inseminate your enemies for a nominal fee, and where suburban moms can escape locked basements like wine-slugging Houdinis. Stranger things have happened.

In tonight's highly anticipated—and highly entertaining—series finale of Pretty Little Liars, we learned that Team Twincer—the fans whose theory that Spencer Hastings has a twin who might just be the show's biggest baddie of them all—were pretty on the money. To recap: Mary Drake had two daughters in Radley. One went to the Hastings family and lived a charmed existence: Spencer. The other was shipped to the U.K. for a more hard-knock life: Alex Drake. (Or—wait for it—A.D.) Enter that smarmy instigator Wren. Years later, he discovers Alex in a London bar and draws her into the liars' web. She developed a relationship with Charlotte (her half-sister), and decided to avenge her death. How? As the liars would say, by stealing the game from Charlotte herself, imprisoning Spencer in a gargantuan underground bio-dome, secretly usurping her life, and taking her place in Rosewood. It's a twisted and ultimately tragic plot that doesn't go as planned, when the girls band together to take down one last A.

When Troian Bellisario runs all this down for me over the phone, from a secure location, after many NDAs have changed many hands, she's still on edge. "It's so unnerving! It feels so weird talking about it," she exclaims. "Like, 'Oh God, am I going to get into trouble?'" Over seven seasons, Bellisario has embrace the bonkers fun of the show while striving to root her character in reality. "We have these fantastic archetypes," she says. "Why not flesh them out and investigate them?" Now, Bellisario is playing the ultimate soap trope—and it's no wonder she still threw herself into it with the kind of powerful, overachieving gusto that she and Spencer (and Alex) share.

All the while, Bellisario has kept a close eye on fans too: Yes, Alex's obsession with Aria is a nod to Team Sparia, she tells me. When I mention a Tumblr user named The Outlast, whose highly detailed and not far-off theories have lit up the PLL fandom more than once, she pauses for a beat. "I think Marlene actually sent me a post by The Outlast a while ago!" Oh, and the reason I. Marlene King, the evil genius behind the show, has been sending her fan theories? Because the two of them have been sharing the secret of Uber A's identity for years. Yes, years.

So when it came time for Bellisario to finally spill some secrets, she didn't disappoint. She told ELLE.com about the day she found out Uber A's identity, how King crafted it, and what it was like to finally film the big reveal.

First thing's first: How did you find out you were playing Uber A?

I was first told in the beginning of season five. Marlene took me aside and told me, "I have an idea, but I don't know if the network will let me do it..." Then she sat me down and explained this whole ending to me. She said, "You cannot tell anybody. It might not even happen." So I sat on it for over a year. Then at the end of season six, she said, "We're going to do it." It was like the starting gun at the races.

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What was the conversation like when Marlene first told you about her plan?

The conversation actually took place while I was shooting a scene. In between takes I would come back to Marlene and sit down cross-legged to hear more. It was Story Time with Marlene! I wanted her to tell me everything. It took two hours to go through the entire pitch. There was so much for her to tell! I didn't even know about Mary Drake—we hadn't really introduced that character's story yet. So I didn't know about Spencer being adopted either. She had a lot to tell me.

So at that point, did it seem like she had this plan all fully formed in her mind?

Yeah. Originally, the show was only supposed to last five years. So she was going to take it through to Charlotte being revealed as A, and that was it. But around season four, the network contacted her and said they really wanted to take the show through to seven seasons. That's when she had to start creating this whole other world, with a new ultimate villain. By the time she let me in on it, she had been thinking about it for at least a full year and had really fleshed it out. Over the next two years, we got to build it together.

Had Marlene been dropping breadcrumbs to support this story during that time?

No, I think it was more fortuitous. The whole arc of Spencer going to Radley and feeling at home there, having a connection to the place, feeling like she had a history there. Maybe it was fortuitous, or maybe it helped lead Marlene to this idea, but those parts of Spencer's story fit perfectly into the revelation that Radley is actually where Spencer was born, that she is a part of its history.

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How did you keep the identity of Uber A a secret for so long?

It's been real rough. But for a long time, people would ask, "Who's A.D.?" And I'd say, "It's me!" And everyone would just laugh. It's kind of great when you have a secret that so few people—only me, Marlene, and a handful of others at that point—know. It means you can hide in plain sight.

What about filming the last episode—was there heightened secrecy on set?

Actually, there didn't need to be a lot of secrecy because we shoot mostly on a backlot. It might be possible for the studio tours to see us filming as they were going through the lot, but I was dressed up as Spencer, and they wouldn't have gotten close enough to hear the way that I was speaking. So they wouldn't have known anything.

You directed your first episode this season. It also happened to be one that offered the biggest clue as to the identity of Uber A, in a scene where [Ian Harding's character] Ezra runs into "Spencer" at the airport. Eagle-eyed fans realized that there was no way Spencer could have gotten to the airport so quickly from the Lost Woods, where she was in her previous scene in the episode. Did you mean to tip your hat to the audience?

Totally. It was the first scene in which Alex gets caught in a situation not of her making, in which someone mistakes her for Spencer. In every other scene, she's very prepared. She walks in pretending to be Spencer; everybody is expecting her to be Spencer. But when Ezra happens upon her and [Julian Morris' character] Wren, it's very much not a part of her plan. We knew that the scene would be a huge tipping point for certain members of the audience. We all agreed that the people who are going to get it are going to get it. I said to Marlene, "We aren't going to be able to control this secret until the very end, but that doesn't mean that the audience isn't going to have fun." For some people, the reveal of Spencer's twin will be a total slap upside the head, and it'll be great. For other people, it'll be fun to feel ahead of the game. It doesn't take the fun out of the mystery. It makes you feel like you're a part of it.

For some people, the reveal of Spencer's twin will be a total slap upside the head, and it'll be great.

In the scenes throughout the season where you were secretly playing Alex, how did you act as that character without giving too much away?

As we were building the evolution of how good she would get at impersonating Spencer, we had to walk that razor-thin edge between keeping the secret and subtly letting the audience know. One of the first scenes where you see Alex is when she shows up as Spencer and asks [Keegan Allen's character] Toby for the kiss goodbye. We were all sweating bullets over what she was going to wear to look like Spencer, how good she would be at imitating her, how uncomfortable she might be, how much it might throw her off. But the fans interpreted it as Spencer being nervous about saying goodbye to Toby. It was wonderful! They filled in all the blanks.

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Did the actors you were playing opposite as Alex know who you were?

Actually, one of the only other people who knew early on, who was on set the day Marlene and I first spoke about Alex, was Keegan. So he and I were kind of the secret-keepers. I don't think the girls knew until well into filming this season. There were a few scenes that I played with them where they didn't know I was playing a different character. They must have just been like, "Huh. Troian's off today!" But by the time the finale rolled by, everybody was a part of it.

What were their reactions when they found out?

There was a scene in the very beginning of the season where [Ashley Benson's character] Hanna wakes up to a dream of younger Spencer talking to her in the cell A.D. put her in. When we came out to the cast about Alex, Ashley said, "Wait, you were a different person in that scene and you didn't even tell me?" I said, "I wasn't allowed to tell you!" She was like, "Man, I'm feeling really betrayed right now!" [Laughs]

What was it like when everyone got the script for the finale?

We had a massive table read in the same room where we'd had our very first table read of the show's pilot script. We sat around the same table and every single writer, every single producer was there, and so many people whose characters came back for the first time in a long time. We all got to sit down and read through the script, almost like a radio play. It was fun to read all the scenes between Alex and Spencer, where Alex is explaining everything that's going on to her twin. It was a total blast to play these two characters in a room, in front of everybody, to see how it was going to go over.

Did you do the English accent?

Oooh, yes. I've never been more nervous—my heart was pounding the whole time. Switching between the accents and the two different personalities, I've never been more exhausted. By the end, I turned to someone else like, "What just happened?"

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How would you describe the essential difference between playing Spencer and Alex?

Spencer comes out from under things, that's where her confidence comes from. Alex has a very different kind of confidence. She's brash, she's, uh, a very sexually confident person. [Laughs] I think of it like this: Spencer approaches things with her head and her heart, and Alex approaches things with her head and her groin. That's not to say she doesn't have a heart or feelings, but she's gotten really good at putting a thick coat of armor over her heart.

What was it like playing an evil English twin secretly playing her American sister?

At first I had to build the history of this character, her idiosyncrasies, and her movements with Marlene. I got to work with an incredible dialect coach to find the character's rhythm. Once you lock that in, it becomes a game of figuring out how good is she at transitioning her personality into Spencer's. Even if you've been observing somebody for a long time, your ability to imitate them is based a lot on your perception and judgements of them. It's not necessarily just about Alex being an incredible mimic. It's also about what Alex thinks of Spencer.

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This is one of the only dramas on TV where ending with the reveal of a long-lost evil twin makes perfect sense tonally. Did you accept and embrace the utter insanity of the show from the beginning?

No, I totally fought against it at first. I beat my head against the Pretty Little Liars machine for a very long time. Then I had a conversation with Norman Buckley, a director who's probably directed more episodes of the show than anyone else. He said, "The circumstances may be fantastical, but the emotions are real." I think a lot about stories that are science fiction or fantasy. If your characters live in a world with dragons and magical powers, or in a world where we can easily travel through space and time, then your job is to play it for keeps. At first that was tough for me because I saw our show more like Gossip Girl in a way. I thought of the characters as just a bunch of people interacting with each other in the real world. But then you start to see that A is everywhere and sees everything, and that it's not physically possible for a single person to do all that. So…let's just say that person has an insane amount of money and the best surveillance equipment in the world! Once you accept those circumstances, then you get to just play in that sandbox.

Spencer approaches things with her head and her heart, and Alex approaches things with her head and her groin.

How the hell did Uber A pay for all the surveillance and equipment and bribes, not to mention the subterranean doll house, anyway?

Well, Wren is incredibly wealthy, and Charlotte left Alex everything. Easy! [Laughs] From the very beginning of the show, we've just had the wealthiest villains and they've continuously passed their insane wealth on to the new villains.

Oddly, the reveal of an evil twin makes more sense in the world of Pretty Little Liars than the revelation that Dan Humphrey was Gossip Girl. In the more realistic—though stylized—world of that show, it was hard to believe that Dan had been terrorizing himself all those years. There was no Radley, no multiple personalities, no evil twin to explain that behavior away.

Yeah, you could roll your eyes, like, "Oh, God, we're doing an evil twin?" But it's so rare to get to tell a story that's serious in one moment and totally wackadoo in the next. And you're playing in a universe where it's not only accepted—but it's expected. As an actor, it's my job to give a performance that's as grounded and real as possible. But in the world of this show, if I'm given the opportunity to play an evil, British twin, then I'm going to do it and have a blast. When else do you get to do that?

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How does it feel now that the world finally knows who Uber A is?

I'm just nervous about how the hell this character is going to go over. It's bold! But even if you figure out that Spencer has a twin, you're never going to guess the backstory and the insanity that brought her to Rosewood. So I think there are still going to be surprises. I also feel like you're not going to get through seven seasons of this twisty-turny, topsy-turvy show, then see me spin around as a different person with a different accent and be like, "No, no, I don't buy it." We've made these great leaps and unexpected turns, and the audience has come with us every time. So I have a lot of faith that the audience is just going to enjoy it. And that's what I hope for them. If it's the last two hours you may ever get to spend in Rosewood, then God, I hope to shit you enjoy it.