Matt Damon‘s advice for Josh Hartnett while filming Oppenheimer came a little too late.

The Trap actor, who played nuclear physicist Ernest Lawrence in Christopher Nolan’s Oscar-winning film, recently stopped by The Tonight Show Starring Jimmy Fallon where he shared Damon’s “unhelpful” acting advice.

“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 noted that the Air star, who portrayed Lt. General Leslie Groves in Oppenheimer, has a general rule when taking on 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. 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 Paradise Lost actor said. “And he kept telling me, like, over the course of the production.” 

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

Damon would know something about undergoing dramatic weight transformations for movie roles. He lost between 40-50 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! 

As a result of the quick weight loss for Courage Under Fire, the actor has previously opened up about health issues he faced at the time.

“I went too far,” Damon has said. “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.”