Matt Damon shared his golden rule for movie roles with Josh Hartnett while they were working on 'Oppenheimer' together.

Hartnett, 46, joined the cast of Christopher Nolan’s Oscar-winning film playing nuclear physicist Ernest Lawrence with Damon, 53, portraying General Leslie Groves, director of the Manhattan Project , and Hartnett has revealed his co-star gave him some "unhelpful" advice by advising him never to gain weight for roles after the age of 40.

During an appearance on 'The Tonight Show Starring Jimmy Fallon', Hartnett explained: "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.

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

The actor added that Damon told him: "Don’t gain weight over 40".

Hartnett went on: "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.

"I was like, ‘Thanks, Matt. Thanks for telling me this now. I’ve already gained it'." He then joked: "And now I don’t eat anymore".

Damon has previously opened up about how changing his weight for roles made him sick. He shed pounds to play a troubled war veteran in 1996 movie 'Courage Under Fire' and the diet left him struggling.

During an appearance on 'The One Show', he explained: "I went too far. 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."