Josh Hartnett Recalls Matt Damon’s Weighty Advice For ‘Oppenheimer’

Josh Hartnett visited The Tonight Show on Wednesday, revealing the “unhelpful” acting advice that Matt Damon imparted about playing nuclear physicist Ernest Lawrence in Oppenheimer.

“He gave me a lot of good advice,” recalled Hartnett. “One in particular — one thing that was just so unhelpful: he told me not to gain the weight I’d already gained for the role. I gained about 30 pounds for the role, and he was like, ‘You’re never gonna get that off again, man.’”

Damon’s general rule of thumb is “don’t gain weight over 40” for any role, Hartnett related.

“He’s like, ‘You’re gonna spend the rest of your life trying to get that weight off, and it’s never gonna come off because your body’s gonna want to get that weight back on. You’re just gonna keep growing back out to that size, and you’re going to try and get it off, but it’s just gonna go back,’” he said. “And he kept telling me, like, over the course of the production.

“I was like, ‘Thanks, Matt. Thanks for telling me this now. I’ve already gained it.’”

Damon once dropped down to roughly 139 pounds in order to play a drug-addicted war veteran in 1996’s Courage Under Fire. He also gained weight for his lead role in 2009’s The Informant!

Now, Damon relies on body prosthetics to portray larger body types.

Watch Hartnett’s take above.