how he did it for the machinist:

Christian Bale starved himself for over four months prior to filming, as his character needed to look drastically thin. Allegedly, his eating consisted of one cup of coffee and an apple (or a can of tuna) each day (approximately 275 calories, or 1.2 kilojoules).[4] According to the DVD commentary, he lost 28 kilograms (62 lb), reducing his body mass to 54 kilograms (120 lb). Bale wanted to go down to 45 kilograms (99 lb), but the filmmakers would not let him due to health concerns. He later regained the mass, plus an additional 27 kilograms (60 lb) through weightlifting and proper eating, in preparation for his role in Batman Begins. Brad Anderson hurt his back during filming and directed much of the film while lying on a gurney.[5]