"I was watching people's hands for three months after," Gyllenhaal added. "It took a long time."
Although the two actors share a familiar chemistry on-screen, getting to that point took work. When they weren't training, they would run lines over and over in Ayer's office or simply hang out to try to build a rapport.
"To be honest, I don't think we got along expertly in the beginning," Peña said. "We had some rough patches."
Eventually, Ayer said, they clicked. "I think it was the acting work that bonded them, and then all the training and going through this common experience," he said. "And then after a while it was like, 'Oh God, guys, shut up!' "
Underscoring the film's snapshot aesthetic, Ayer shot "End of Watch" largely on the streets, in a pseudo-documentary style. Much of the action was captured by Gyllenhaal on a hand-held digital video camera, as his character is working on a documentary for a school project. Other scenes unfold before clip-on cameras mounted on the officers' vests, dashboard cameras and cellphones.