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 – o…