How 'Pretty Little Liars' Star Troian Bellisario Hid That Huge Finale Secret in 'Plain Sight'

After seven long years, "Pretty Little Liars" fans finally found out who Uber A is, but one star has been struggling to keep it top secret long before the series finale aired Tuesday night.

The episode revealed Troian Bellisario's character, Spencer, had a long-lost evil twin that was the mastermind behind Rosewood's stalker. The actress told Elle Magazine she's known about it since Season 5 and has been hiding Uber A's identity from the rest of the cast.

"I was first told in the beginning of Season 5. Marlene took me aside and told me, 'I have an idea, but I don't know if the network will let me do it,'" Bellisario said. "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 6, she said, 'We're going to do it.' It was like the starting gun at the races."

"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."

When Bellisario learned that the conclusion of the series would end in her having an evil twin, she said at first it seemed a little silly, but then realized that "Pretty Little Liars" might be the only show on television that could pull something like that off.

"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," Bellisario said. "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," she continued. "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.

"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," she added.

As for finally being to revel the identity of Uber A, Bellisario said she's curious to see how people will react.

"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."