Josh Hartnett reveals 'unhelpful' advice Matt Damon gave him on “Oppenheimer”: 'Thanks, Matt'

The actor said that Damon repeatedly told him he'd made a mistake by gaining weight for the Christopher Nolan film.

Josh Hartnett wasn’t prepared for the bombshell advice Matt Damon dropped on him while filming Oppenheimer.

While visiting The Tonight Show on Wednesday, the actor revealed the “unhelpful” acting advice that Damon imparted upon him when it came to playing nuclear physicist Ernest Lawrence in the Oscar-winning Christopher Nolan film.

“He 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 host Jimmy Fallon. “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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As Hartnett tells it, Damon’s general rule of thumb is “don’t gain weight over 40” for any role. 

“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 recalled. “And he kept telling me, like, over the course of the production.”

Hartnett could only take Damon’s delayed advice in stride. He teased, “I was like, ‘Thanks, Matt. Thanks for telling me this now. I’ve already gained it.’”

Damon is no stranger to undergoing weight transformations for a role. He once dropped down to roughly 139 pounds in order to play a drug-addicted war veteran in 1996's Courage Under Fire and, over a decade later, gained weight for his lead role in 2009’s The Informant! 

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In the years since, Damon has utilized body prosthetics to portray larger body types onscreen, most recently in the 2023 film, Air. The film's director, Damon’s pal Ben Affleck, told Entertainment Tonight that they not only “put Matt in a body suit,” but also “shot him with a lens that expanded his face a bit.” 

“I’m so accustomed to seeing Matt Damon as the kind of Jason Bourne, can’t lose, gonna win no matter what,” the filmmaker said. “And I wanted to try to create more of that sort of underdog feeling.” 

Watch Hartnett recall Damon’s advice in the clip above.

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