Josh Hartnett shares ‘unhelpful’ advice Matt Damon gave him on Oppenheimer set

Hartnett and Damon starred alongside each other in the Oscar-winning biopic

Josh Hartnett has shared the “unhelpful” advice that Oscar-winner and co-star Matt Damon shared with him on the set of Oppenheimer.

The Christopher Nolan-directed movie, swept up seven awards out of 13 nominations at the 2024 Oscars. It chronicles the career of American theoretical physicist J Robert Oppenheimer and his role in the development of the world’s first nuclear weapons during the Second World War.

The Pearl Harbour star played Ernest Lawrence, a Nobel Prize-winning nuclear physicist who worked with Oppenheimer at the University of California in Berkeley.

“He gave me a lot of good advice,” began Hartnett on the Tonight Show with Jimmy Fallon. “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 portrays General Leslie Groves, a United States Army Corps of Engineers (USACE) officer and the director of the Manhattan Project, the name given to the research and development initiative to develop the world’s first nuclear weapons, resulting in the first atomic bomb.

Hartnett explained that the Good Will Hunting actor has one piece of advice that he follows when taking on any new roles, “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,’” Hartnett continued.

‘Oppenheimer’ won seven Academy Awards (Universal Pictures/Oppenheimer)

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

The Paradise Lost star joked, “I was like, ‘Thanks, Matt. Thanks for telling me this now. I’ve already gained it,’” Hartnett replied. “And now I don’t eat anymore.”

Oppenheimer went on to gross more than $1billion at the box office upon its release in 2023.

Nolan secured his first Academy Award as Best Director for the nuclear biopic, having been nominated five times previously for films including Dunkirk (2017), Inception (2010), and Memento (2000).

He kept his acceptance speech mostly industry-focused, thanking his stars, who are “all at the top of their game” and his “incredible crew” and reserved a special shout out for his wife, Emma Thomas, who has worked as a producer on all of his films since 1997.