Sir Alfred Hitchcock convinced Sir 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.
Based on W. Somerset Maugham's "Ashenden" spy stories ("The Traitor" and "The Hairless Mexican") and a play by Campbell Dixon.
When Peter Lorre's character becomes highly agitated, it isn't necessarily due to acting. At the time of filming, he was receiving treatment for drug addiction and was experiencing withdrawal symptoms.
Sir Alfred Hitchcock reflected (regarding Sir John Gielgud's lack of heroics): "You can't root for a hero who doesn't want to be one."
Sir John Gielgud filmed this during the day while appearing on-stage in "Romeo and Juliet" opposite Dame Edith Evans, Dame Peggy Ashcroft and Sir Laurence Olivier in the evening.