It was a tall order for the young star -- who had just come back after a devastating accident on the set of Maze Runner: The Death Cure, which required months of recovery -- and he says the rigorous stunt work made him seriously apprehensive about taking the part.
"The state that you're in after something like that, you just want to run from all that stuff," O'Brien told ET's Leanne Aguilera at the American Assassin press junket on Monday, referring to the dangerous stunts and high-energy fight scenes in action films. "I didn't really even want to leave my couch necessarily when I was allowed to, so yeah, it was a big step doing this."
"I ultimately decided that I wanted to try it and I didn't want to let it go," he added. "[The movie] ended up being really instrumental in the whole process."
While filming The Death Cure in March 2016, O'Brien was shooting a stunt atop a moving vehicle when, despite a safety harness, he was thrown from a vehicle and struck by another vehicle, causing serious damage to his face. The actor suffered a concussion and several facial lacerations.
According to co-star Michael Keaton, who joined O'Brien at the junket, everyone on the American Assassin set was very conscientious of maintaining a safe working environment, and they were careful not to do anything that would inhibit O'Brien's full recuperation.
"We had to go through the choreography for the [fights] because I was aware [of his previous injuries]," Keaton said. "I didn't want to talk to him about it or bug him about it or remind him, but I had to know how close he could get, how close he can't get, what side do I flip him on, because I don't want him to hit that area. So it made it harder, but in a way, it was better. You had to be very specific about every move you made."
"It forced you to go about it the right way, which was good," O'Brien added.
In American Assassin, O'Brien plays Mitch Rapp, a young man who dedicates his life to hunting and killing terrorists after his girlfriend is gunned down in a terror attack. Keaton stars as Stan Hurley, a Cold War veteran spy who puts Rapp through a grueling training regimen and teaches him to become a precision hit man. Together, they team up to hunt down a rogue operative intent on starting a nuclear war.
Despite his initial apprehension, the 25-year-old star said he "really loved and was very fascinated with all the fight training and the gun training" he underwent for the film.
"At the time that I came in, and what I was coming off of, I was still at the very end of recovery there when I started, so that was probably the biggest challenge. All the training was very good for my mind and very good for recovery, in a way. It became part of my daily schedule," O'Brien added. "It really ended up being [therapeutic]. It came at a great time for me."
"Me too, because I was looking for someone to beat the s**t out of," Keaton joked.
"Our goal is to go in and finish it with a bang," he said. "I think we did good work, we had a really good crew down there in South Africa, a really good experience and [it would be] really good for me to put it away on a nice positive note, too. So yeah, it's exciting. I'm excited about it."