During the scene where jake gyllenhaal talks to himself in the mirror, gyllenhaal got so into the scene that the mirror cracked, cutting his hand. he had to go to the hospital for 14 stitches, returning to the set after being discharged.
jake gyllenhaal lost 20 pounds for his role. this was gyllenhaal's own idea, as he visualized lou as a hungry coyote.
to create lou's gaunt appearance, jake gyllenhaal worked out for up to 8 hours a day and ran or biked to the set everyday.