First part of the "Trilogy of Neurosis", also including The Working Class Goes to Heaven (1971) and Property Is No Longer a Theft (1973).
During post-production, Elio Petri and his editor pranked Ennio Morricone by showing him an alternate edit of the opening scene with a completely different score. Morricone was visibly upset, at which point Petri apologized and said "You've composed the best score ever, you should punch me in the face for pulling a stunt like this!". This was revealed by Morricone himself in a posthumously released documentary.
The main character's name is never revealed.
Cannon had hoped to remake this in the mid 1980s with Sidney Lumet or Andrei Konchalovsky directing. Al Pacino and Christopher Walken were to star, and Paul Schrader would have been the screenwriter.
When Dottore (Gian Maria Volontè) goes to visit "il questore" (Gianni Santuccio) for the first time (around 37 mins), there is a poster hanging on the wall with a portrait of Gabriele D'Annunzio, the Italian poet linked to the fascist regime and personal friend of Benito Mussolini, who coined the term "L'Orbo Veggente", the one-eyed visionary, after he lost one of his eyes in 1916. D'Annunzio was not just a poet, but also very much involved in esotericism.