The Jarhead star spent months patrolling the mean streets at night before filming on the gritty new movie began, and he admits the experience gave him a real taste of what police in Los Angeles have to put up with.
And his very first night on the beat was one that really tested his nerve.
He tells WENN, "The very first ride-along that I went on someone was murdered in front of me. It was a gang shooting that was obviously drug related. We were the second car on the scene and I had no idea what to expect when I went on the first ride-along. I was scared at the time because it's really easy to become complacent.
"You show up for the 5pm to 5am shift which is a strange time of night to be working and you don't get a call until 3.30 in the morning. All of a sudden you get a call for a shooting and you have to deal with that and keep your mind in check.
"What was actually scariest was watching these police officers and asking myself, 'How will I function being an actor in the back seat of their car, which is an extra responsibility for them, to make sure we're safe and protected?' You got some pretty incredible guys dealing with both things at the same time. It was really life affirming.
"My three closest friends are police officers from this adventure and that's more important to me than any movie."