The film was shot in reverse continuity so Christian Bale, having worked hard to lose weight for the role, would appear the gauntest at the end, and then could simply gain the pounds back over the course of filming, working backwards through time so that when Bale returned to his average weight he would be filming his scenes as Dengler prior to being taken prisoner.
Dieter Dengler was captured not once but twice in real life. The scene where he was captured while drinking from a river is based on his second capture.
In a show of solidarity, as many of his actors lost weight for their roles, the director lost almost 30 pounds.
All the guards in the POW camp were based on true characters from Dieter Dengler's story except for 'Walkie Talkie' who is a fictional creation of the director.