Josh Hartnett revealed during an interview on “The Tonight Show” (via Entertainment Weekly) that Matt Damon advised him on the set of Christopher Nolan’s “Oppenheimer” not to gain any weight for his role. The only problem is that Hartnett had already packed on the pounds to play Nobel Prize-winning nuclear physicist Ernest Lawrence. Damon starred in the film as Manhattan Project director Leslie Groves. 

“[Matt] 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 said. “I gained about 30 pounds for the role, and he was like, ‘You’re never gonna get that off again, man.’”

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According to Hartnett, Damon’s rule of thumb is that you just “don’t gain weight over 40” for any movie role because it’s near impossible to shed the pounds. 

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“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 concluded.

Regardless of whether or not Hartnett needed to gain weight for the role, the actor received widespread acclaim for his supporting turn in the Christopher Nolan-directed drama and won a SAG Award as part of the movie’s ensemble cast. “Oppenheimer” went on to win several Oscars, including best picture and best director.

“Oppenheimer” earned $976 million at the worldwide box office, giving Hartnett a career resurgence that is set to continue with the release of M. Night Shyamalan’s new thriller “Trap.” The actor also popped up as a guest star in the third season of FX’s Emmy-winning “The Bear.”

Watch Hartnett’s full interview on “The Tonight Show” in the video below. “Trap” opens in theaters Aug. 2 from Universal.

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