'Pretty Little Liars' star on that shocking series finale twist

— -- Last night's series finale of "Pretty Little Liars" came with a huge twist.

SPOILERS AHEAD

The girls had a run-in with the new Uber A (aka A.D.), only to find out it was Spencer Hastings' twin sister Alex Drake, both played by actress Troian Bellisario.

In an interview with Elle.com, Bellisario spoke about the big reveal, when she found out the major twist and what it was like being the show for seven seasons.

"I was first told in the beginning of season 5. Marlene [King, the show creator] 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," the actress said. "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.'"

As for how she kept the secret for so long, she said she did the first thing that came to mind.

"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," she said.

Now Bellisario is happy to see this "bold" plot choice revealed to the world.

"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," she added. "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,'" she continued. "So I have a lot of faith that the audience is just going to enjoy it."

On last night's finale, it was explained that Alex took over the game as Uber A when Charlotte, Spencer's half-sister, died. Alex felt like Spencer had it all, like amazing friends and family, while she was left with nothing and grew up in an orphanage in England.

Alex is eventually apprehended by the end of the episode and taken to jail for her crimes.