Josh Hartnett in "Oppenheimer."UniversalJosh Hartnett gained 30 pounds for his role in "Oppenheimer."His costar Matt Damon warned him it would be difficult to lose the weight after filming wrapped."He was like, 'You're never gonna get that off again, man,'" Harnett recalled Damon telling him.Matt Damon knows a thing or two about the challenges of gaining and losing weight for a role.