EXCLUSIVE: 'Pretty Little Liars' Author Spills Her Latest 'A' Theory: 'I Don't Trust Tanner!'

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It's no secret that Pretty Little Liars is one of the biggest hits on television.

It’s addictive, it’s mysterious, and it's all thanks to the woman who penned all 16 of the PLL books: Sara Shepard.

To celebrate tonight's "biggest reveal" in PLL history, ETonline sat down with the pretty little author to discuss why she loves not knowing the show's deepest secrets, her latest 'A' theories, and which PLL couple has always been her favorite!

It's a common misconception when fans think that Shepard has any clue about who's hiding under 'A's hoodies on the show, because she purposefully chooses to be out-of-the-know. "I just want to watch with everyone else I just like not knowing," she spilled to ETonline. "It's fun to really genuinely be surprised at all of the twists and turns and to have theories and share them with all the fans and listen to their theories."

NEWS: 'PLL' Boss Talks Big 'A' Reveal: 'Oh My God — 'A' Has a Soul!'

Pretty Little Liars executive producer Marlene King has promised that tonight's finale will be a "shock and a revelation" when fans discover a huge clue about 'A's identity. Despite not knowing the truth, Shepard thinks she may have cracked the case and she thinks the infamous Twin Theory might be involved.

"Lately after live-tweeting I've been recapping the episodes too and that was the last thing I was talking about," she said. "I think maybe if it's not a twin of Ali, it's got to be a secret DiLaurentis something or other."

ABC Family

"I also do not trust Tanner," she added, "I don’t know how Tanner could be 'A', but I will say Tanner can get into a prison and can kind of movie freely through certain worlds."

When asked to name her favorite pretty little couple, Shepard was torn between a few swoon-worthy pairings. "I loved Aria and Noel in the books and I loved Hannah and Mike and they're two relationships that are not in the show at all," she said of the book-based twosomes.

WATCH: 'PLL' Boss Drops Major 'A' Clues - 'You May See Double!'

As for her favorite PLL couple on the show, Shepard has swayed between two fan-favorites. "Another couple that I really liked on the books and in the show was Ali and Emily -- Emison," Shepard explained with a smile. "For a while on the show I went through a 'I don’t like Ali phase,' I was like, 'Don’t trust her!' and I wanted Emily to be with Paige."

Shepard continued, "But now I loved the phone call in the last episode, I love that Ali called Emily from prison. I just loved writing about them. They're weird like Emily knew that it was wrong, but Ali knows that Emily is weak for her, and I loved writing about it."

Speaking of writing, Shepard's book have been huge small screen hits for ABC Family. The Lying Game ran for three years as a drama and Pretty Little Liars has been renewed through season seven. Now one of Shepard's newest book series The Perfectionists, could be coming to ABC Family as well.

"I am very very fortunate," Shepard said when asked about the secret to her success. "I think they're all thrillers, they're all edge of your seat stories and I think the characters are all really relateable. I just try to make everything really real and funny and soapy and scary -- that sort of seems to be my thing."

For more exclusive PLL insight from Shepard, watch our full interview in the video above!

Pretty Little Liars season five finale airs tonight at 8 p.m. on ABC Family.

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