Josh Hartnett Reveals ‘Unhelpful’ Advice Matt Damon Gave Him on Oppenheimer

Josh Hartnett received some “unhelpful” advice from his co-star Matt Damon while filming Oppenheimer.

What was the Advice Matt Damon Gave to Josh Hartnett While Filming Oppenheimer? 

During an interview on The Tonight Show Starring Jimmy Fallon, Hartnett spoke about playing nuclear physicist Ernest Lawrence in Christopher Nolan’s Oppenheimer. Hartnett gained over two dozen pounds to play the role. Hartnett might not have put on the extra pounds for Oppenheimer had he spoken with Damon earlier.

“He [Damon] gave me a lot of good advice. 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,” Hartnett told Fallon. “I gained about 30 pounds for the role, and he was like, ‘You’re never gonna get that off again, man.’”

Hartnett broke Damon’s rule of thumb for acting: don’t gain weight over 40.

“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,’” Hartnett 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.’”

Hartnett, Damon, and the Oppenheimer cast won many accolades for Oppenheimer, including the Outstanding Performance by a Cast in a Motion Picture at the SAG Awards. Oppenheimer went on to dominate awards season, winning seven Oscars, including Best Picture.

Hartnett and Damon are both starring in movies that come out in August. Hartnett is the lead in M. Night Shyamalan’s new thriller Trap, which bows in theaters on August 2. Damon stars opposite Casey Affleck in Doug Liman’s The Instigators, which streams to Apple TV+ on August 9, 2024.