Matt Damon kept telling Josh Harnett he'd regret gaining 30 pounds for 'Oppenheimer:' 'You're never gonna get that off again'

Josh Harnett gained 30 pounds for his role in "Oppenheimer," but his costar Matt Damon told him during filming that it was a mistake.

• Josh Hartnett gained 30 pounds for his role in "Oppenheimer."
• His costar Matt Damon warned him it would be difficult to lose the weight after filming wrapped.
• "He was like, 'You're never gonna get that off again, man,'" Harnett recalled Damon telling him.

Matt Damon knows a thing or two about the challenges of gaining and losing weight for a role. Too bad he didn't warn his costar Josh Harnett before he decided to gain 30 pounds to play the famed physicist Ernest Lawrence in "Oppenheimer."

"He was like, 'You're never gonna get that off again, man,'" Harnett recalled Damon saying on "The Tonight Show."

"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."

The fact that Damon's advice was too little too late wasn't lost on Hartnett. "He kept telling me, like, over the course of the production," he added. "I was like, 'Thanks, Matt.'"

Damon knows about the dangers of gaining and losing weight for roles from his own experience. After losing 40 pounds to play a PTSD-stricken soldier in 1996's "Courage Under Fire," a doctor told him the dramatic change could have permanently shrunken his heart.

Years later, he gained 30 pounds for the 2009 satire "The Informant!" He's also done intense training for the Jason Bourne movies, following a 2,000-calorie-a-day diet and cutting back on water intake to show off his physique in 2016's "Jason Bourne."

But if the actor, now 53, follows his own advice — Hartnett recalled him saying, "Don't gain weight over 40" — then perhaps Damon's days of on-set transformations are over.