In law enforcement, "end of watch" has two meanings; it commonly refers to time to go off duty at the end of shift (some agencies call shifts "watches"). Also, if an officer is killed in the line of duty, the date of his death is referred to as his end of watch.
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."
The scene roughly at the end of the movie where Mike and Brian are in pursuit of a silver minivan and then crash into it 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 removed the anti-lock brakes from the patrol car because it was shaking the camera and what the actors also didn't know was that the road that they were filming the scene on 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.
The scene where Janet and Brian are singing in the car during a road trip was unscripted. Jake Gyllenhaal and Anna Kendrick were driving around with the director in the back seat filming different scenes for the movie. At one point between takes, 'Hey Ma' by Cam'ron came on the radio and Gyllenhaal and Kendrick started to sing along and the director began to secretly film them. Both Gyllenhaal and Kendrick were completely unaware that the moment had been caught on camera until they watched the movie at a screening.
To achieve the shots captured by the main characters' lapel cameras, Michael Mansouri of rental house Radiant Images stripped down a Silicon Imaging SI-2K Mini camera to about a third of its original size, to be attached to special vests worn by Jake Gyllenhaal and Michael Peña. The camera was dubbed the "Nano", and was subsequently made available to other productions for rental.
When Taylor ('Jake Gyllenhal') keeps tugging on his shirt collar area to pull it down, he's actually adjusting his bulletproof vest. The vest is semi-rigid and rides up on you when seated in a car. Officers tug down on the vest often to make adjustments for comfort and fit.
In the beginning briefing sequence, as the officers get out to their cars, everyone keeps yelling out, "Faster, BOOT" in reference to the rookie officer Sook. In Law Enforcement lingo, "BOOT" is term for a rookie or new officer.
When Zavala mentions "Badge Bunnies" to Taylor, he is talking about women who like cops for being a cop, reasons can include the uniform, the badge, or whatever comes with the job. It's a true phenomenon and every station has their local badge bunnies... and a warning to stay away from them.
Jake Gyllenhaal and Michael Peña were given taser electro-shocks as a part of their research, as it is required in police training. Contrary to rumors that the entire cast was tased, Anna Kendrick claims to have abstained, and said she didn't think any of the other actors should have agreed to it either.
In the house full of kidnapped people, statues of Santa Muerte can be seen. Santa Muerte is a sacred figure commonly venerated primarily in Mexico and the United States by people involved in various drug cartels and other criminal enterprises.
In the house where the officers find all the human trafficking people is an altar with small statues and likenesses of Jesús Malverde. Jesús Malverde, sometimes known as the "generous bandit", "angel of the poor", or the "narco-saint", is a folklore hero in the Mexican state of Sinaloa. Though he is celebrated as a folk saint by some in Mexico and the United States, particularly among those involved in drug trafficking, he is not recognized as a saint by the Roman Catholic Church.