Donald Pleasence had actually been a Royal Air Force pilot in World War II, who was shot down, became a prisoner of war and was tortured by the Germans. When he kindly offered advice to Sturges, he was politely asked to keep his "opinions" to himself. Later, when another star from the film informed Sturges that Pleasence had actually been a RAF Officer in a World War II German POW Stalag camp, Sturges requested his technical advice and input on historical accuracy from that point forward.
James Garner had been a soldier in the Korean War and was twice wounded. He was a scrounger during that time, as is his character Flt Lt Hendley.
Richard Attenborough was an RAF air gunner/photographer who served in the RAF for three years unlike his character, based on Squadron Leader Roger Bushell who was a Spitfire Pilot in 92 Squadron in the early years of World War Two.
Several cast members were actual P.O.W.s during World War II. Hannes Messemer was held in a Russian camp and Til Kiwe and Hans Reiser were prisoners of the Americans.
Steve McQueen walked out demanding rewrites and John Sturges considered writing the character out of the story altogether but United Artists informed him they considered McQueen indispensable to the picture's success and financed a new writer to accommodate his demands. This led to a diminished role for Big X, who was supposed to be the main protagonist.
The most notable of McQueen's demands was for Hilts to have a scene on a motorcycle, which ended up being one of the most iconic moments of the film.
Early on in the production, Sturges began receiving memos from distributor United Artists requesting female roles in the picture. One even suggested having the dying Ashley-Pitt cradled in the lap of a beautiful girl in a low-cut blouse. The studio wanted to cast this bit by having a Miss Prison Camp contest in Munich. Sturges would have none of it.
Wag the Director: The motorcycle chase was added at the insistence of Steve McQueen, who signed onto the film on the condition that he show off his biking skills. During filming, McQueen held up shooting by demanding his character be given more to do.
Richard Harris was originally cast as Roger Bartlett, but dropped out because filming This Sporting Life was behind schedule and he was displeased with the diminished role of Big X after script changes had been made.
John Mills turned down the role of Roger Bartlett.