Quiz

Dario Argento's usual collaborator Ennio Morricone scored the film, but had a major argument with Argento over some tracks Argento didn't want in it. As a result, the director and Morricone would not work together again until 1996, and the rock group Goblin would eventually become Argento's regular composers.

Deep Purple was originally considered for the soundtrack.

Bud Spencer took the role of Diomede/Godfrey in the film as a favor for Dario Argento, who he became life-long friends with during the shooting of two Westerns the latter had written, Oggi a me... domani a te! (1968) and Un esercito di 5 uomini (1969). He did not take a fee for his services, but asked to not have his likeness used to promote the film.

The Italian poster for Dario Argento's earlier film L'uccello dalle piume di cristallo (1970) can be seen in a street shot as Roberto drives through the city.

A slow-motion bullet effect is used in the film, pre-dating Matrix (1999), as well as many Hong Kong action films.