It has been alleged that Tony Musante, known for his intensity and preparedness as an actor, would frequently show up at Dario Argento's apartment at 3 am to discuss characterization, much to Argento's annoyance.
The painting the killer buys from the antique store heavily resembles Pieter Bruegel The Elder's: "The Hunters in the Snow."
Although Das Geheimnis der schwarzen Handschuhe (1970) isn't the first Giallo movie, it is credited for making the sub-genre famous and starting the trend that boomed from 1970 to 1975.
This film played in a Milan cinema for three and a half years; such was the film's popularity in its native Italy.
At one point, an executive producer wanted Dario Argento removed from the production when he was disappointed by a screening of some dailies. When Argento's father Salvatore Argento went to the exec's office to talk to him he found the exec's secretary visibly shaken. When he asked the secretary what was troubling her she said she saw the screening and the footage terrified her. Salvatore Argento then asked her to go tell her boss about her reaction to the screening. She convinced the executive to keep Dario on as director.
Dario Argento: body doubling as the killer. Every shot of those black, leather gloves, as it was far more convenient for him, knowing exactly how to handle the hand movements.