It's time for A.D. to reveal everything!

During Tuesday night's series finale of Pretty Little Liars, Troian Bellisario sent shockwaves through the fandom when the ultra-mysterious identity of A.D. was revealed to be Spencer Hastings' long-lost identical twin, Alex Drake. (Gasp!)

Now that the series is officially over, Bellisario is opening up to ET about all the behind-the-scenes secrets that she's been keeping for years -- including the original way PLL was supposed to end, Spencer and Toby's romantic future, and which pretty little cast member had the best reaction to A.D.'s unveiling!



MORE: 'Pretty Little Liars' Series Finale: The 8 Biggest Questions Finally Answered!



ETonline: Troian! We haven’t chatted since the day of your PLL directorial debut when we told you our spot-on theory about Spencer having an identical twin! What was going through your mind at the time?



Troian Bellisario: I know! Do you remember my face? [Laughs] You were so vocal and talking about other things and then you were like, "OK, so here’s what we think is happening…" and I was like, looking at you and I was thinking, "Oh no! Please stop. Stop talking. Stop talking because I'm not going to be able to confirm or deny this." Like, what was I supposed to say? [Laughs] Like, "Wow great theory, uhh ..." But it turns out, that was entirely accurate.

Take us back to the conversation when PLL showrunner Marlene King first told you that A.D. was going to be Alex Drake, aka Spencer's identical twin. What was your reaction to the news?



TB: Basically, I'm so nosy and I really like knowing all the storylines before they all come out, because I love to be a part of them. And I did this for everybody, like, "Tell me about what's going on with Aria! Tell me what's going on with Hanna! What's next for Emily?" And I said to Marlene, "What's ultimately going to happen to Spencer?" And she was like, "I could tell you what I ultimately think is going to happen to Spencer, but it would take a bit of time." And I was like, "What else am I doing? I'm just on set. Tell me the plan." And then she proceeded to tell me the entire ending plot of the show and I was like, "Wait -- what?! Oh my god!" and then I had this moment where I was like, "Wait, you're not going to be hiring someone else to play my twin though, right?" And she was like, "No it would be you!" And I was like, "Yes!" But it was just so much, and when she told me it was a crazy turn of events, but I trusted that she would layer it in beautifully. It was a total blast.

Which cast member had the craziest and most exciting reaction when he/she found out that you were also going to be playing A.D.?



TB: It's always Ashley [Benson]. Ashley is just the greatest. Even last night, we were watching it together -- and she was at the table read, and she filmed this episode with me -- but she was always leaning over and goes, "Is that you or your twin?" And by "you," she meant Spencer. It was so funny because she just kept saying, "Is that you or your twin?" And I was like, "That's the twin," and she'd go, "Oh my God! So creepy!" And then she'd be like, "Is that you or your twin?" And I'd be like, "No, that one is me -- that’s Spencer," and she'd go, "Oh God! OK!" But it was just so much fun to watch the episode with the cast and to dress up. I got to bring my mom [to the screening] and in the first moment that the twin was revealed, it was so funny. She turned around at me and she gasps and goes, "You bitch!" and Marlene basically fell off her chair laughing. It was so funny! And I was like, "But, Mom! I'm also the person being held captive!"



WATCH: Here's Everything the 'Pretty Little Liars' Cast Has Spilled About the Series Finale!



Let's talk about that accent! What was the decision to give Alex Drake that specific dialect of a British accent?



TB: It was funny because it was something that we were all super nervous about, because I'm obsessive when it comes to accuracy. I don’t want to ever do an approximation of something, and the people on our show who are British like Julian [Morris] and like Huw [Collins] they have very beautiful and very erudite white London accents. So when Marlene told me the story of Alex, I was like, "I've got to tell you, if this girl is a working girl and she has had, like, a rough upbringing, I don’t want her to be speaking like how Wren speaks. I want to be very clear from the beginning that she's had a different life than Wren has had." To me, the best way to communicate that was to give her a very specific accent, so I really wanted to learn the Essex accent. Essex girls are kind of fun, and there's this reality show called The Only Way is Essex and and once you hear that cadence, and you listen to the way that these girls talk to each other when they get excited or passionate, I was like, "That's this character. I can base a character on that." What I was a little bit afraid of was when Marlene said that she's going to be from the U.K., I was like, "OK here we go. Here comes the British bad guy, like, a James Bond villain or something like that." So I was like, "I kind of want her to be something unexpected." And because Spencer is so sort of refined and poised and slow and methodical, I really wanted Alex to vibrate on a different level. It was fun getting to work on the accent, but it's been totally nerve wracking during filming because people were like, "Wait, people really talk like that?" And I was like, "Yes, yes they do go watch that show."

What was your favorite scene to play when you were Troian playing Alex playing Spencer?



TB: When I was Alex pretending to be Spencer? Oh gosh, It was really fun in all the finale scenes, that was great, but probably the cabin scene with Toby. It was a few episodes back when she goes up and they end up sleeping together, that was the most fun to play.

What little mannerisms did you give to Alex that Spencer didn’t have?



TB: Oh! Well because of her accent -- this is going to sound so nerdy -- but we Americans have a very flat, kind of broad "A" sound. If you look at the way that we use our mouths, it's a little bit more wide. Alex is so fast and so quipped and she's really colorful, but if you notice there's a lot of tension around her mouth, so even when she's impersonating Spencer, I really wanted to kind of have her struggling with that exact kind of freedom in the American accent. There's just a lot of tension around her American accent and literally sort of a stiff upper lip. Then there was also something that was really fun that she does that I got to kind of play with this season, which is Alex is a little bit of a nail biter. She's kind of constantly trying not to do that. But it was really fun and I was actually surprised with how it all turned out. I was really worried that there wasn't going to be enough of a distinction between them, but I was really of happy to look back and sort of be like, "Yup!" I can totally see the differences, particularly in the finale, where I'm like, "Yup that's Alex and that's Spencer." It was so fun.



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Let's talk about the final scene where it's revealed that Mona is keeping Alex and Mary Drake hostage as her living dolls. What was your reaction to reading that scene and how do you feel about the ending now?



TB: I loved it! It was so fitting and it was so fun to film. And that was actually our last day of filming and there was nothing that felt like a more fitting way to end Pretty Little Liars then to be trapped in a dollhouse, in a tea dress, and grumpily eating a crumpet. It was just really fun. There was actually an amendment to the ending. Originally -- I don’t know if I'm supposed to say this -- but I think originally the ending was supposed to be a little bit different where Alex had kind of gotten away with it a little bit more, and the only person that was onto her was Toby. But I think that ultimately would’ve left things too much up in the air and we would've been worried for Spencer. So it was really fun because this way, Spencer got a happy ending, and all the girls got a happy ending, and Alex got to be with her family - which is great, but also in a little bit of a tricky situation.

Speaking of happy endings, can you talk about Spencer and Toby? When we left off, they were on good terms and it was revealed that they were starting to date again, so in your mind, where do you envision they end up?



TB: I mean, I always felt like Spencer and Toby were going to end up being just like, the most badass domestic partners. I always felt like if they were to get married, it would be such a [quiet] elopement that the girls would be mad. Almost like the way that Hanna and Caleb did, because they didn’t tell anybody and then it happened and then they were married. So I feel like the cool thing about Spencer and Toby is, they're such independent characters and their relationship is so nontraditional that I feel like they would just end up dating but for the rest of their lives. They would have children together, and they would live together, but they would also be like, "We are independent people." If he needs to go backpack through the Himalayas for year, he could totally do that. And if she needs to go live on her own in Paris for a year, she could totally do that. But I think that they would always end up coming back to each other.

And have family Scrabble nights together?



TB: Oh yeah, those poor kids! [Laughs] Those poor nerdy kids!



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Now that Pretty Little Liars is officially over, what are you going to miss most about playing Spencer and Alex?



TB: Well with Alex, it was really just the tip of the iceberg. It was so much fun to get to create an entirely new character, but then you want to live in her. You want to relax in her. I sort of feel like watching Alex last night was very similar to the way that I felt watching my performance as Spencer in the pilot. When I saw myself as Spencer in last night's episode, there was just such easiness and it was a fully fleshed out, three-dimensional character that you lived with and people have gotten used to over the last seven years. Every move that she makes is very in her skin. But Alex, like it was with Spencer in the first episode, is a very shiny new car. When you're trying to change gears in it, it was very different. It's got a lot of energy and speed that you're not used to. It's fun and it was so much fun to play both of the characters, but I wish that I could have more time to really stretch Alex's legs and sink into her a bit more. And then what am I going to miss the most about playing Spencer? I mean, Spencer is hands down one of the coolest characters I've ever had a chance at playing and I can't believe that I will never play her again. Like, that's the weirdest. That's the thing I'm going to miss the most.

How are you feeling about Pretty Little Liars' ending? Were you a fan of the A.D. reveal? Which Liar are you going to miss the most? Share your thoughts (for one last time!) with our pretty little expert @LeanneAguilera on Twitter!