Dyeing for Your Art: Jake Gyllenhaal actually did shave his head for the film.
Impersonating an Officer: Done unintentionally toward the real-life LAPD. During the scene where Taylor and Zavala stop a runner, an LAPD patrol helicopter was flying overhead and stopped to check out what was going on. This was not part of the script. Luckily, thanks to the amount of preparation Gyllenhaal did to portray a police officer, he was able to use a hand signalnote four fingers = "Code Four, no further assistance needed" to tell the chopper crew that everything was OK, with them none the wiser.
Throw It In!: Ayer wanted Gyllenhaal and Peña to actually "live" their lives as two close friends who just happened to be LAPD officers. A lot of their bantering is mostly the two ad-libbing.
