Pretty Little Liars star, sterne Keegan Allen: I Know Who 'A' Is ... and So Do You! 

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Keegan Allen, who stars as Spencer Hastings\'s (Troian Bellisario) boyfriend Toby Cavanaugh, says he has the answer ... and meticulous viewers could solve the mystery, too.

"I do know who it is," Allen tells PEOPLE of the latest baddie to cyber-bully Rosewood\'s four leading ladies. "And what I love about who it is is that everyone knows who it is ... but it will be so shocking and surprising."

because it\'s so clearly in every episode," he adds, laughing. "When it\'s finally revealed, you could literally go back to season 1 and knowing who it is, you can be like, \'Oh, wow. Whoa. This has been here all along, and I never noticed it,\' which is, I think, brilliant."

Challenge accepted. But as we obsess over the ABC Family series, Allen is pursuing another one of his many passions: photography. The 25-year-old\'s new photo book,

From taking his first ever picture (of the Los Angeles riots) at the age of 10 to snapping behind-the-scenes shots of his costars, the book chronicles Allen\'s creative journey and rise to fame.

"Before I ever began my training as an actor, I was really obsessed with the idea of sharing moments," he says, "and the best way to share moments – and I think one of the most compelling means of human communication – is photography."

was the third most popular show on Twitter for 2014, it\'s no surprise that Allen is using social media to promote the project. But even Tumblr queen Taylor Swift hasn\'t gotten

intimate with followers: Keegan is sending Snapchats to and even calling people who pre-order

"The response has been so incredible, and I\'ve gotten to talk to hundreds and hundreds of people, and a lot of people have shared their own personal photo journals," he says. "I don\'t look at my fans as fans and followers, I look at them as friends."

As for his day job, Allen kept coy about whether the Liars can trust his character in a town where every cop seems to go crooked ("That\'s a question for sure for [showrunner] Marlene King and [executive producer] Joseph Doughtery") but promises plenty of twists and turns.

"So much of the show separates itself from other teen dramas out there because [the writers] have knowledge of classic cinema," he says. "They\'re showing the audience all of the cards this season, they\'re giving the audience all the secrets."

And for Tuesday night\'s episode, Allen is teasing a big scare.

"I do remember shooting a scene and going to our producers like, \'You guys are going to really do that, okay, cool, wow ... that\'s terrifying, but okay,\' " he says. "I was taken aback by how extreme tonight\'s episode is."

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