Warning: Major spoilers ahead!

The moment Pretty Little Liars [1] fans had been waiting for for years finally happened on Tuesday night during the show's shocking series finale [2]: Uber A is revealed [3], and her identity is a whammy. It turns out the fan-favorite theory that Spencer Hastings (Troian Bellisario) has an evil twin [4] named Alex Drake has been correct all along, and that Alex has been posing as Spencer throughout Rosewood for a few episodes now. The big revelation was a WTF moment for a lot of people watching the show, so Elle decided to go right to the source and see how Bellisario first found out Spencer had an identical twin [5] and get her opinion on how PLL creator I. Marlene King decided to wrap up Spencer, Aria, Hannah, Emily, and Alison's stories.

• On Finding Out She Would Be 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."
• On How She Learned About A.D.: "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."
• On How She's Kept This 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."
• On the Epic Table Read: "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."
• On How PLL Almost Ended in a Completely Different Way: "Originally, the show was only supposed to last five years. So [Marlene] was going to take it through to Charlotte being revealed as A [6], 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."
• On Being Worried About Fans' Reactions: "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 sh*t you enjoy it."