 The physical trainers also had to ensure that not only do the actors look like soldiers before filming , but they had to remain that way throughout the shoot . In addition , Stapleton and Winchester had to lose and gain weight , respectively . Winchester had to be on a diet of 4 @,@ 000 calories a day , which included taking protein shakes between meals . The actor described the diet as " a workout in itself , " having found it " exhausting " eating more food than he was used to . In total , training took approximately one month . 
