Along with Repulsion (1965) and Rosemary's Baby (1968) this film is part of a loose trilogy by Roman Polanski dealing with the horrors faced by apartment/city dwellers.
Cameo: [Philippe Sarde] the man that stares at Trelkovsky in the movie theatre.
Although Roman Polanski plays the leading role in the film, he is given no screen credit as an actor.
Philippe Sarde (the composer) chose the glass harmonica after having seen Polanski, at the restaurant, mimicking with his finger the action of making the glass sing. There was only one person left in the world that could play this instrument, for which Mozart wrote a few pieces.
One of the ten most terrifying moments in history of cinema in the opinion of the French horror movie magazine Mad Movies.