Josh Hartnett reveals the ‘unhelpful’ advice he received from Oppenheimer co-star Matt Damon: ‘Thanks for telling me this now’

Josh Hartnett is currently promoting his new film Trap, the latest psychological thriller from famed director and screenwriter M. Night Shyamalan.

But during his appearance on The Tonight Show on Wednesday, the actor spoke about his time on the set of the 2023 blockbuster Oppenheimer.

Hartnett specifically revealed the “unhelpful” advice he received from co-star Matt Damon.

“He gave me a lot of good advice. One in particular — one thing that was just so useless: he told me not to gain the weight I had already gained for the role,” Hartnett said.

Hartnett gained 30 pounds to play the role of Nobel Prize-winning nuclear physicist Ernest Lawrence in Christopher Nolan’s acclaimed film.

“I had gained about 30 pounds for the part, and he was like, ‘You’ll never lose that, man,’” he recalled.

It turns out that Damon, now 53, told him he has a rule: you can’t gain weight for a role if you’re over 40.

“He said something like, ‘You’re going to spend the rest of your life trying to lose those pounds, but you’re never going to succeed because your body wants to put those pounds back on,’” he recalled.

‘You just keep growing back up and you try to get it off, but it just comes back.’

The star of the Bourne film franchise not only told him this at the beginning of production, he continued to reiterate his stance on weight “throughout the entire production.”

This was a mystery to Hartnett, who responded by saying, “Thanks, Matt. Thanks for telling me this now. I already got it.”

In the Oscar-winning film, Damon played General Leslie Groves, an officer in the United States Army Corps of Engineers (USACE) and director of the Manhattan Project.

Acclaimed Irish actor Cillian Murphy played the title role of Robert Oppenheimer. The film focuses on his time working on the Manhattan Project, the World War II project that developed the first nuclear weapons.

The story also shows how the famous physicist struggled during World War II to finally create the world’s first atomic bomb.

In addition to Hartnett, Damon and Murphy, the cast also includes major stars such as Emily Blunt, Florence Pugh, Robert Downey Jr., Rami Malek and Kenneth Branagh, as well as Benny Safdie, Michael Angarano, Dane DeHaan, Dylan Arnold, David Krumholtz, Alden Ehrenreich, Casey Affleck and Matthew Modine.

Oppenheimer received widespread critical acclaim, particularly for the direction, screenplay, acting, cinematography, editing, music, and visual effects.

The film was nominated for a whopping 13 Academy Awards, winning seven, including Best Picture, Best Director, Best Actor for Murphy and Best Supporting Actor for Robert Downey Jr., Best Original Score, Best Cinematography and Best Film Editing.

Critics weren’t the only ones to express their appreciation for Oppenheimer. Moviegoers also made the film a smash hit at the worldwide box office, grossing $976.1 million against a budget of $100 million.

Hartnett is currently promoting his new starring role in Trap, M. Night Shyamalan’s latest psychological thriller, which hits theaters August 2.

Hartnett leads the cast, which also includes Ariel Donoghue, Saleka Shyamalan, Hayley Mills, Alison Pill, Marnie McPhail, Vanessa Smythe and Kid Cudi.

In Trap, Hartnett plays the role of Cooper, a serial killer also known as The Butcher. He goes to a concert of pop star Lady Raven with his daughter. He realizes that this is a trap set by the police to catch him.