John Gielgud filmed this during the day while appearing on stage in "Romeo and Juliet" opposite Peggy Ashcroft and Laurence Olivier in the evening.
Alfred Hitchcock convinced John Gielgud to play the lead by describing the hero as a modern day Hamlet. Gielgud, however, ended up hating that his character was an enigma and felt Hitchcock made the villain more charming than the hero.
Alfred Hitchcock reflected (regarding John Gielgud's lack of heroics): "You can't root for a hero who doesn't want to be one."
Based on W. Somerset Maugham's "Ashenden" spy stories ("The Traitor" and "The Hairless Mexican") and a play by Campbell Dixon.