Some of the actors had been POWS in the Second World War. Denholm Elliott, while serving in the RAF, had been shot down and taken prisoner by the Nazis.
Dirk Bogarde was offered the extended cameo of Smedley-Taylor.
In Interviews Bryan Forbes has said he had to fight the Screen Actors Guild over the most of the 15 British Equity members he wanted to cast. The Screen Actors Guild wanted British SAG members most who were fairly elderly most not suitable for the film.
SPOILER: In the scene where King (George Segal) and his inner circle of fellow prisoners are cooking the dog, the actors (other than Segal) were not told what they were supposed to be eating in the scene. Their reactions are authentic and ad libbed.