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 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. For years the identity of "the man who never was" was a closely guarded secret, until it was discovered he was called Glydwr Michael. His grave in Spain uniquely carries both his fictional and real names.
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.
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."