Matt Damon Gave Key Advice To Oppenheimer Co-Star Josh Harnett At The Worst Time Possible

Actor Josh Hartnett has been around for quite awhile, breaking hearts as a teenager in films like "The Faculty" before going on to have a prolific career with quite a bit of range. One might imagine that because of that amount of experience, he's heard every bit of acting advice out there. However, it turns out that Matt Damon had some new advice for him when they worked together on Christopher Nolan's historical drama "Oppenheimer," and it came at the worst time possible.

While on "The Tonight Show" (via Entertainment Weekly), Hartnett shared a story about filming with Damon and revealed that the "Good Will Hunting" actor had some advice that came a bit too late, which made it pretty darn moot. Thankfully, he said that Damon also gave him plenty of great acting advice and that he learned a lot from their time working together — except, again, for just that one thing...

Hartnett plays nuclear physicist Ernest Lawrence in "Oppenheimer," a role for which he gained 30 pounds. Apparently Damon told him that he shouldn't have gained the weight for the role, but it came after the fact. As Hartnett explained: 

"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.' And he kept telling me, like, over the course of the production."

While it seems like Hartnett managed to get the weight back off just fine if his shirtless scene in M. Night Shyamalan's "Trap" is any indicator, it's pretty funny that Damon kept harping about it after Hartnett had already gained the weight. Damon's insistence on not massively changing his body is well-documented, as he used a body double for "The Martian" instead of gaining and losing weight. Still, maybe he should have told Hartnett before he had put on 30 pounds? Then again, Hartnett wasn't the only one who massively changed their diet for Nolan's historic opus.

While not everyone makes quite as big of a spectacle as Rob McElhenney gaining 60 pounds in three months for a season-long gag on "It's Always Sunny in Philadelphia," actors gaining and losing weight for roles is pretty commonplace. While no one else on "Oppenheimer" acquired mass quite like Harnett, star Cillian Murphy barely ate anything while filming in order to mimic the lifestyle of Oppenheimer during his trial. He became rail-thin and wan, and while it's easier to gain weight again after a period of fasting, that's also probably not very healthy!

Damon is actually onto something, even if his delivery wasn't great. Major body changes for roles can really wear on a person, and even the king of gaining and losing weight for roles, Christian Bale, finally had to quit doing so because it was detrimental to his health. Getting into a role and really giving it your all is impressive, but here's hoping more actors take Damon's advice and prioritize their well-being before diving into major weight change.