To commit to the film, the 33-year-old actor shed nearly 30 lbs. in order to fully inhabit his character: a skeevy paparazzo named Lou Bloom whose insatiable appetite for grisly news scoops veers toward moral depravity.
It was a visual and thematic metaphor not lost on Gyllenhaal, who revealed to Entertainment Weekly that he stripped his diet to a mere combo of kale salad and chewing gum as he transformed his body into a near-ghoulish echo of itself.
"I made a lot of choices physically," he said, noting that he ran 15 miles a day to and from his home in the Hollywood Hills and the film's set. That grueling physical regimen eventually took its toll on his body – and his social life.
"The running thing, you're pretty hungry because you're not eating a lot of food," he says. "You're lonely because you're not meeting your friends for dinner. People go, 'Hey you want to meet for dinner after work?' I go, 'Well, I'm shooting all night.' 'All right, you want to meet for lunch?' I'm like, 'I can't!' So I'm gonna go run."
Looking back now, Gyllenhaal is astonished at the lengths to which he pushed himself for the film, both physically and mentally.
"In terms of my face and the choices that were made, in terms of losing weight and stuff, that was just months of getting to it," he later told PEOPLE while promoting the film on Saturday in Los Angeles. "Just running ... 15 miles a day. I'm just picturing myself as a coyote with all the coyotes and stuff. And my face just changed, I think."
"I don't think I was aware," he added of how deep he'd plunged into character. "I don't think I was aware until a few months ago and we were going through all the cuts, and you start to separate from the character and go 'Wow,' you know?"
With reporting by MIKE MILLER