Brian Truitt

USA TODAY

Audiences haven’t had the chance to see how Jack Bauer, Jason Bourne or James Bond became secret agents. But with American Assassin, they’ll witness the birth of an action hero in the origin story of young CIA counterterrorism specialist Mitch Rapp.

Vince Flynn’s literary character comes to life onscreen for the first time in Assassin, now filming in London (with a release date to be determined). Maze Runner star Dylan O’Brien plays the man trained to become a government assassin after his fiancée is killed in a terrorist attack.

“You always wonder who these people are, where they come from, how’d they get to what they’re doing,” O’Brien says. “A lot of the time, there is a past and a tragic trail and a traumatic experience. It’s a really cool look into human nature.”

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The geopolitical thriller opens with post-grad student Mitch proposing to his true love “before everything changes in his life drastically,” says director Michael Cuesta (Kill the Messenger).

A year and a half later, Mitch is a CIA black ops recruit assigned to training officer Stan Hurley (Michael Keaton) by Deputy Director Irene Kennedy (Sanaa Lathan). The pair is tasking with tracking down a missing cache of weapons-grade plutonium, exposing a global terrorism network and disarming a nuclear weapon — all while Mitch is figuring out the balance between the need to feed his personal bloodlust and real injustice.

“There’s an internal battle, absolutely, in terms of the pain he holds inside and how to correctly process that rather than just becoming a ruthless murderer,” says O’Brien.

That allows for a realistic look into the secret-agent life. “What does it actually feel like to have your first kill?” producer Lorenzo di Bonaventura asks.“We explore that in an emotional context, not just a ‘We got the bad guy’ plot context.”

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Keaton brings complexity, unpredictability and even warmth to Hurley, Cuesta says. “He’s tough and borderline sadistic at times, but there’s also a tenderness. He is showing this young man the right path, even though he has a rough way of going about it.”

Iranian-born actress Shiva Negar co-stars as a Turkish agent who works with Mitch and Hurley, and Taylor Kitsch plays the mysterious villain, a former government operative known by a variety of names — though none Cuesta is giving up yet.

Kitsch’s bad guy is a dark reflection of what Mitch could become if not for Hurley’s tutelage: While Mitch buys into retribution, his foe is destroyed by his need for revenge, says di Bonaventura. “There’s a scene where (Kitsch's character is) going to do a nefarious thing and he’s singing God Bless America. It really messes your head up when you hear it.”

Filming Assassin has been a highlight in a “really, really rough year” for O’Brien: An accident on the set of the third Maze Runner movie in March left the 25-year-old actor with a concussion and broken bones.

“To be honest, it’s the biggest challenge of my life coming out here and starting up on this,” he says. But he’s “proud” to be at the center of a potential action-movie franchise. “It took me a while to get there, but I’m doing good.”