Jake Gyllenhaal lost 20 pounds for his role. This was Gyllenhaal's own idea, as he visualized Lou as a hungry coyote.
During the scene where Jake Gyllenhaal screams at himself in the mirror, Gyllenhaal got so into this improvised scene that the mirror broke, cutting his hand. He was driven to the hospital by the director after a nineteen-hour day of working and got forty-six stitches in a four-hour long operation, returning to the set six hours after being discharged. This is the reason why Gyllenhaal had his hand behind his back in the scene where he tells the scrapyard owner his motto.
Jake Gyllenhaal and Riz Ahmed rode along with actual "nightcrawlers" in Los Angeles to prepare for their roles.
To create Lou's gaunt appearance, Jake Gyllenhaal worked out for up to 8 hours a day and ran or cycled to the set everyday.