During a recent appearance on The Tonight Show, Josh Hartnett shared an anecdote about Matt Damon's 'unhelpful' advice while filming Oppenheimer. Hartnett had gained about 30 pounds to portray nuclear physicist Ernest Lawrence, but Damon cautioned him against this, claiming that losing the weight would be impossible as one ages. Damon, known for his own weight transformations for roles, advised Hartnett that after 40, gaining weight becomes a lifelong struggle, a sentiment Hartnett humorously acknowledged, noting he had already gained the weight by the time Damon offered his advice. Despite this, Hartnett's performance in Oppenheimer earned him significant acclaim, contributing to a career resurgence highlighted by upcoming projects. The film itself was a box office success and won multiple Oscars, including Best Picture.