Michael Pena co-stars in the film about the long-term friendship and partnership between two police officers. The two of them spent five months working the streets of Los Angeles with police partners and the sheriff's department. They worked a 12-hour night shift three times a week to get into character as crack cops.
Gyllenhaal also spent months getting into shape, but he met his match at a Kenpo karate school in Los Angeles.
He explained to WENN, "Five days a week we would go to Echo Park to this dojo run by the best friend of David Ayer, the writer/director, and Michael and I would fight these 14 to 20-year-old kids. I don't even have a fight scene in the movie but the director wanted me to get into that attitude of what it was like to get beaten up or get into a fight.
"We simulated that every morning so I was constantly sore and in pain getting hit in the face and body. So, for five months before I started shooting, I had the crap beat out of me."
The cast also got into character by getting painfully zapped by a stun gun.
"The experience on the street in the south end of Los Angeles with the police officers that do this job every day of their lives was incredibly intense and it changed my life and put everything into perspective," Gyllenhaal added.