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

Josh Hartnett is currently out 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 working on set of the 2023 blockbuster Oppenheimer.

Specifically, Hartnett 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 unhelpful: He told me not to gain the weight I'd already gained for the role,' said Hartnett.

Hartnett put on 30 pounds to play Nobel-winning nuclear physicist Ernest Lawrence in the acclaimed Christopher Nolan film.

During an appearance on The Tonight Show this week, Josh Hartnett revealed the 'unhelpful' advice he received from Oppenheimer co-star Matt Damon on set of the 2023 blockbuster; Hartnett seen on July 24
Damon seen in February

During an appearance on The Tonight Show this week, Josh Hartnett revealed the 'unhelpful' advice he received from Oppenheimer co-star Matt Damon on set of the 2023 blockbuster 

'I gained about 30 pounds for the role, and he was like, "You're never gonna get that off again, man,"' he recalled.

It turns out Damon, who is now 53, told him he has a rule to not sign off on gaining weight for a role over the age of 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," he recalled.

'"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."'

The Bourne film franchise star didn't just tell him at the beginning of the production, he also kept reiterating his stance on weight 'over the course of the production.'

This was a head-scratcher for Hartnett, who responded by telling him, 'Thanks, Matt. Thanks for telling me this now. I've already gained it.'' 

For the Oscar-winning film, Damon portrayed General Leslie Groves, a United States Army Corps of Engineers (USACE) officer and director of the Manhattan Project. 

Acclaimed Irish actor Cillian Murphy played the title role of Robert Oppenheimer, which focuses on the time when he worked on the Manhattan Project, the World War II undertaking that developed the first nuclear weapons.

Hartnett put on 30 pounds to play Nobel-winning nuclear physicist Ernest Lawrence in the acclaimed Christopher Nolan film
'[Damon] 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,' said Hartnett

Hartnett put on 30 pounds to play Nobel-winning nuclear physicist Ernest Lawrence in the acclaimed Christopher Nolan film. '[Damon] 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,' said Hartnett

For the Oscar-winning film, Damon portrayed General Leslie Groves, a United States Army Corps of Engineers (USACE) officer and director of the Manhattan Project

For the Oscar-winning film, Damon portrayed General Leslie Groves, a United States Army Corps of Engineers (USACE) officer and director of the Manhattan Project 

Oppenheimer received widespread critical acclaim, including for Cillian Murphy's Oscar-winning portrayal of the titular character

Oppenheimer received widespread critical acclaim, including for Cillian Murphy's Oscar-winning portrayal of the titular character

The story also shows how the famed physicist wrestled with the ultimate creation of the world's first atomic bomb during World War II. 

Along with Hartnett, Damon, and Murphy, the cast also includes such big stars as Emily Blunt, Florence Pugh, Robert Downey Jr., Rami Malek and Kenneth Branagh, along with the likes of Benny Safdie, Michael Angarano, Dane DeHaan, Dylan Arnold, David Krumholtz, Alden Ehrenreich, Casey Affleck, and Matthew Modine.

Oppenheimer received widespread critical acclaim, with particular praise towards its directing, screenplay, cast performances, cinematography, editing, musical score, and visual effects. 

Among its many accolades, It received 13 Academy Awards nominations, winning seven of those including, Best Picture, Best Director, Best Actor for Murphy and Best Supporting Actor Rober Downey Jr., Best Original Score, Best Cinematography, and Best Film Editing.

The critics weren't the only ones to show their love for Oppenheimer, as moviegoers helped the film become a bonafide hit at the worldwide box office, grossing $976.1 million while working with a budget of $100 million. 

Hartnett is currently out promoting his new leading film role in Trap, the latest psychological thriller from M. Night Shyamalan; seen together on July 24

Hartnett is currently out promoting his new leading film role in Trap, the latest psychological thriller from M. Night Shyamalan; seen together on July 24 

For Trap, Hartnett plays Cooper, a serial killer dubbed The Butcher, who joins his daughter at a concert for pop star Lady Raven, an event he realizes is a trap set by police to catch him

For Trap, Hartnett plays Cooper, a serial killer dubbed The Butcher, who joins his daughter at a concert for pop star Lady Raven, an event he realizes is a trap set by police to catch him

Trap hits theaters on August 2

Trap hits theaters on August 2

Hartnett is currently out promoting his new leading film role in Trap, the latest psychological thriller from M. Night Shyamalan that's set to be released in theaters August 2.

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

For Trap, Hartnett plays Cooper, a serial killer dubbed The Butcher, who joins his daughter at a concert for pop star Lady Raven, an event he realizes is a trap set by police to catch him.