As part of their training, Jake Gyllenhaal and Michael Peña spent five months doing 12-hour ride-alongs with on-duty LAPD officers. During Gyllenhaal's first ride-along, he witnessed a murder.
The scene near the end of the movie where Mike and Brian are in pursuit of a silver minivan and then crash into it: the crash was not scripted. Michael Peña, who was driving the patrol car at that moment, actually crashed into the back of the minivan by accident.The director had had the anti-lock brakes removed from the patrol car because they would cause the camera to shake. Also, the actors didn't know that the road on which they were filming the scene had recently been repaved. So when Michael Peña hit the brakes, the patrol car skidded across the road for about a 150 feet right into the back of the minivan deploying the airbags.
In law enforcement, "end of watch" has two meanings. It commonly refers to the end of a police officer's shift. (Some agencies call shifts "watches".) It can also refer to the date of an officer's death if he is killed in the line of duty.
After Taylor and Zavala pull over the male in the pickup who tries to shoot Zavala, Taylor holds up four fingers to Orozco when she asks, "Are you good?" This is often used by police to signal "Code Four," meaning "I'm okay; no additional assistance needed at this time."