- 'Plummer, Christopher' was originally considered for the lead role, but dropped out to star in Sound of Music, The (1965).
- In the Len Deighton novels the name of the lead character is never revealed. 'Caine, Michael' suggests "Harry" and the film's executives put forward "Palmer".
- Palmer is the first action hero to wear glasses (Caine is myopic in real life).
- Michael Caine's Harry Palmer character is depicted as an accomplished cook, but when you see Palmer skillfully break a couple of eggs, the hands in the close-up belong to Len Deighton, author of the book on which the movie is based.
- Producer Harry Saltzman hated director Sidney J. Furie and his oddball style and went so far as to bar him from the editing room. According to Furie, Saltzman also excluded him from the film's party at Cannes and even stole his best picture British Academy Award.
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