Matt Damon's advice to Josh Hartnett while filming 'Oppenheimer' was ‘so unhelpful’

Matt Damon had some advice for his fellow "Oppenheimer" star Josh Hartnett, but apparently it came a little too late.

Hartnett appeared on "The Tonight Show Starring Jimmy Fallon" earlier this week, laughing at Damon’s "unhelpful" suggestion.

"He gave me a lot of good advice," Hartnett said. "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 continued, "I gained about 30 pounds for the role, and he was like, ‘You’re never gonna get that off again, man. Don’t gain weight over 40. You’re going to spend the rest of your life trying to get that weight off and it’s never going to come off because your body is going to want to get that weight back on. You’re just going to keep growing back out to that size, and you’re going to try and get it off, and it’s just going to go back.’ He kept telling me over the course of production."

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In the film, the now 46-year-old Hartnett played Ernest Lawrence, a Nobel-winning physicist who worked with J. Robert Oppenheimer. Damon, 53, played Lt. General Leslie Groves, director of the Manhattan Project.

"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."

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Hartnett’s role is on the smaller side in "Oppenheimer," and he joked about the pressure being off during awards season while supporting the film.

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"It is kind of fun when you’re not up for any awards," he said. I just get to have a couple drinks with people and hang out and watch other people go up there and give speeches and make a fool of themselves and it’s great."

Fallon pointed out Oppenheimer won best picture at this year’s Oscars, and Hartnett said he got to go up on stage when that happened, adding, "If you look at all the pictures on the cover of The New York Times and all that, I was the one just off the edge of frame where they cut it. It was like, they included one of the ushers, not me."