Matt Damon told Josh Hartnett he’d ‘never’ lose 30-pound ‘Oppenheimer’ weight gain

Josh Harnett revealed the “unhelpful” advice he received from his “Oppenheimer” co-star Matt Damon while promoting his new film “Trap” on “The Tonight Show” on Wednesday.

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

Hartnett, 46, explained that Damon, 53, who played Lt. General Leslie Groves in “Oppenheimer” (2023), has a rule when taking on roles: “Don’t gain weight over 40.”

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

Hartnett put on 30 pounds for his role as the Nobel-winning nuclear physicist Ernest Lawrence in the Christopher Nolan film.

Damon’s advice on Hartnett’s weight didn’t stop after one comment but continued “over the course of the production.”

The “Goodwill Hunting” star is known for his dramatic weight changes, previously losing between 40 and 50 pounds in about 100 days to play a drug-addicted Persian Gulf War veteran in 1996’s “Courage Under Fire.” He later gained weight for his role in 2009’s “The Informant!”

However, Damon admitted that the rapid weight loss for “Courage Under Fire” led to health issues.

“I went too far,” the actor admitted to the UK outlet Express. “I got sick and I wouldn’t do that again because it was just too much. At the same time, it helped the performance. I didn’t have to act at all; I was a wreck. I was getting dizzy spells and hot flashes. I didn’t say anything to anyone for a while because I was afraid I might be really ill.”

“Oppenheimer,” which also starred Cillian Murphy, Emily Blunt, Florence Pugh, Robert Downey Jr., Rami Malek and Kenneth Branagh, received widespread critical acclaim. It was praised for its directing, screenplay, cast performances, cinematography, editing, musical score, and visual effects. 

The film earned 13 Academy Awards nominations, winning seven of those, including Best Picture, Best Director, Best Actor for Murphy, Best Supporting Actor for Robert Downey Jr., Best Original Score, Best Cinematography and Best Film Editing.

It was also a major hit at the worldwide box office, grossing $976.1 million while working with a budget of $100 million. 

Hartnett is promoting his lead role in “Trap,” M. Night Shyamalan’s latest psychological thriller. He plays Cooper, a serial killer known as “The Butcher,” who attends a concert with his daughter, only to discover it’s a trap set by the police to finally catch him.

The cast also features Ariel Donoghue, Saleka Shyamalan, Hayley Mills, Alison Pill, Marnie McPhail, Vanessa Smythe and Kid Cudi.

“Trap” debuts in theaters on August 4.