"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.
The run-and-gun approach combined with the actors' training allowed Ayer to shoot quickly, without extensive choreography. "I just send them in and the cameras follow," he said. "It's not this big rehearsal-fast."
If the shoot was a sort of controlled chaos, Ayer was in his element.
"I think Dave is dealing with dichotomies all the time," Gyllenhaal said. "He's dealing with the dichotomy on the street with the fact that he's friends with gangsters and he's friends with police officers. He's madly in love — and I don't misuse the word 'madly' — with both sides of the line."
While considering his future, which includes script work on a new "Scarface" movie and directing the upcoming Arnold Schwarzenegger action film "Ten," Ayer spoke of another dichotomy.
"I would be perfectly happy to make cop movies for the rest of my career on one level," he said. "But on the other level, there's a lot more I want to do."