Trivia
The film's controversial nature led to a prologue being added during post-production, explaining that there was no connection between Mongolism and psychopathic behavior.
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The whistling theme from the score was resurrected in Quentin Tarantino's Kill Bill: Vol. 1. Daryl Hannah's character Elle Driver is whistling it as she enters the hospital.
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Much of the interior house footage was shot on location at the Mills family home in Twickenham.
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Alfred Hitchcock cast both Barry Foster and Billie Whitelaw in his film Frenzy after watching their performances in this film.
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According to a Bernard Herrmann soundtrack album, the whistler's name is Gareth Williams. Gareth Williams is also credited with some work on the orchestration of the piece.
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