Not eating. To not get fat. I lost about 15 pounds for the role because I wanted to get a sense of what it's like to be hungry and what it does to your brain and your physical state. It was extremely tough to not be eating as much.
Hemsworth on discussing most difficult part of preparing for role for The Hunger Games. — Stephen Schaefer (March 20, 2012). "Hemsworth craved meaty part". Boston Herald: pp. 28, 29; volume 30, issue80.