During filming, the actors were given clues as to their next location through messages given in milk crates found with Global Positioning Satellite systems. They were given individual instructions that they would use to help improvise the action of the day. Teeth were obtained from a Maryland dentist for use as human remains in the film. Influenced by producer Gregg Hale's memories of his military training, in which "enemy soldiers" would hunt a trainee through wild terrain for three days, the directors moved the characters far during the day, harassing them by night and depriving them of food.