O'Brien knows what it's like to come out of the other side of a traumatic incident, and he used it to bring this character, based on the one created by Vince Flynn for a series of novels, from page to big screen.
"I felt like I really understood this character in a way more than I ever could have, funny enough," he told CNN.
In March 2016, O'Brien was seriously injured while filming the third film in the "Maze Runner" franchise.
The actor suffered multiple injuries, including a facial fracture, and production on the film was shuttered for months.
O'Brien filmed "American Assassin," arguably the most physically demanding role the "Teen Wolf" actor has had to date, while he was still recovering.
There was a point, however, when he was unsure he'd be able to see his commitment through.
"I didn't know if I could do it for a while," he said. "It was definitely a challenge that...lingered over my head a little bit in my recovery time. I decided I didn't want to pass it up."
O'Brien stars alongside Michael Keaton in the movie, out now.
Keaton plays a no-nonsense operative named Stan Hurley, who takes loose cannon Rapp under his wing.
O'Brien admitted taking the role while still recovering was "not easy" and he had some limitations during production. But they approached training "slowly."
"That actually ended up being really good for me and something I really enjoyed and something that was really good for my mind," he said. "The movie as a whole was a big step for me. It got me back on my feet. So it will actually always be really important to me -- this time, this movie."