The super-serious actor spent months growing a scraggly beard and picked up a wardrobe from a charity store before taking his cardboard sleeping mat to L.A.'s notorious Skid Row.
He tells WENN, "Just living in those clothes helped me, getting my stink in it. It was a great exercise in empathy and being present because everyone is reduced to, 'Well, there's no roof here,' if you're not getting into the shelter that night and it's raining. I was trading cigarettes for Fig Newtons.
"It's tough at night. There's violence there but there's also a beautiful community that tries to take care of their own. You have families sleeping next to families.
"I went out with a piece of cardboard, a knapsack and my wardrobe - bare bones. I was sleeping head to feet along the buildings and I took my experience back to (director) Oren (Moverman) and some things felt right. Like a journalist, you bring it (to) your editor/director and hope they don't cut the s**t out of it and they honour what you brought them."