In the film, Montague selects a man who had died of pneumonia, because the corpse would need to present with similarly damaged lungs if it had really drowned. There is also a very emotional scene where the man's father is persuaded to allow his son to be used for the deception. In fact, a Welsh vagrant, both of whose parents were dead, was used, and his death was due to his committing suicide by ingesting rat poison. It was judged to make it almost impossible to tell that this, rather than drowning, was the real cause of death. The identity of "the man who never was" was a closely guarded secret until 1998, when it was discovered that he was called Glyndwr Michael. His grave in Spain now uniquely carries both his fictional and real names.
Clifton Webb, who usually wore a mustache in his films and in real life, had a full beard in this film because Royal Navy regulations forbade its officers and men from wearing only a mustache. To comport with the Navy's rules, Webb's choice was to be either entirely clean-shaven or to have a beard. The real Ewen Montagu, the character Webb portrays in the movie (and who is seen in a cameo as an Air Marshal in the meeting of the Chiefs of Staff), was in fact clean-shaven.
In addition to providing the voice of Winston Churchill in two scenes of this film, Peter Sellers can also be heard as the voice coming over the airfield tannoy during the parachute demonstration and, very briefly, as the voice of an unseen taxi driver.
The quote that opens and closes the movie, "Last night I dreamed a deadly dream, beyond the Isle of Sky, I saw a dead man win a fight, and I think that man was I" is from the song "The Battle of Otterburn," Child Ballad #161 and appears in a manuscript dated circa 1550. The original reads, "But I hae (have) dreamed a dreary dream, Beyond the Isle of Skye; I saw a dead man win a fight, And I think that man was I."
The real life Gen. Nye objected to a number of lines that the film's Gen. Nye, played by Geoffrey Keen, said in the script. The production team had to have talks so Gen. Nye would not have to be deleted from the script.