Sometimes, memorizing lines is the easy part for actors.

Physical changes can make their jobs quite challenging. Take the formerly-chubby Parks and Recreation star Chris Pratt. He recently admitted he believed his physique might have stopped him from securing a role in Guardians of the Galaxy. While speaking at a press conference in London to promote the Marvel film, he said, “I think that if I could be believed physically as the character, [the role] would be mine.” He took on an intense fitness regime, and looked the super hero part by the time production started.

Pratt joins a long list of weight changers. Rapper 50 Cent lost weight recently to a play a football player with cancer. Natalie Portman lost over 20 pounds for her role in Black Swan. She did cross-training and swimming for five hours a day, as well as practicing ballet for six months.

It’s not always weight loss that’s drastic. Jared Leto gained 67 pounds for his role as Mark David Chapman in Chapter 27. He reportedly ate nothing but unhealthy snacks like microwaved pints of ice cream mixed with soy sauce and olive oil. It actually gave him gout, leaving him in a wheelchair from time to time.

One of the most drastic changes on both sides would be Christian Bale’s yo-yo weight in 2004. He lost 63 pounds for his role in the The Machinist. The following year, he gained all of the weight back—plus 30 pounds—for his role as Bruce Wayne in Batman Begins.

Galaxy hit theaters on Friday. Pratt also has another major role, in the upcoming film Jurassic World. The film picks up where Jurassic Park left off 22 years ago. It’s due out in theaters June 2015.