During a recent appearance on 'The Tonight Show', Josh Hartnett humorously recounted the 'unhelpful' weight gain advice he received from his co-star Matt Damon while filming 'Oppenheimer'. Hartnett gained 30 pounds to portray physicist Ernest Lawrence, only to hear Damon caution him, 'You’re never gonna get that off again, man,' highlighting a personal rule to avoid gaining weight over 40. Hartnett lamented Damon's timing, saying, 'Thanks for telling me this now. I've already gained it.' Damon, known for his own extreme weight fluctuations for roles, once lost significant weight for 'Courage Under Fire', which led to health complications. 'Oppenheimer', featuring a star-studded cast, has been a critical and commercial success, grossing over $1 billion at the box office.