Director Dario Argento's original idea was that the ballet school would accommodate young girls not older than 12. However, the studio and producer Salvatore Argento (his father) denied his request because a film this violent involving children would almost certainly be banned. Dario raised the age limit of the girls to 20 but didn't rewrite the script, hence the naiveté of the characters and the occasionally childlike dialogue. He also put all the doorknobs at about the same height as the actress' heads, so they would have to raise their arms in order to open the doors, just like children.
It is often assumed that, to achieve the rich color palette, the film was shot using the outdated three-strip Technicolor process. This is not true. No film made after the mid-'50s was shot using this method. This film was instead shot on normal Eastmancolor Kodak stock, then printed using the three-strip Technicolor process, utilizing one of the last remaining three-strip machines. This issue has been confused somewhat by the fact that, on the 25th anniversary documentary featured in the three-disc DVD set, a discussion of the printing process by cinematographer Luciano Tovoli was followed by a diagram showing a three-strip camera.
The voice heard whispering on the bizarre soundtrack by Goblin is that of Goblin band member Claudio Simonetti. Simonetti stated in interviews that much of what he whispers on the music score is just gibberish.
According to Jessica Harper, since the film was going to be dubbed after principal photography, sound was rarely recorded during shooting. Harper remarked that it was strange to her to be in the middle of shooting a scene and hearing the background sound of a stagehand hammering away on another set in the studio.
In an interview, Jessica Harper said that many of the actors spoke different languages during shooting, mostly Italian and German, and it would make communicating difficult at times. However, since the film would be dubbed into English for American release, it was deemed not to be an issue during filming.
Originally the film was to have starred Daria Nicolodi, who was Dario Argento's girlfriend at the time and who also wrote the screenplay. However, Argento decided to go with a younger actress. Nicolodi does appear in the film twice: she can be glimpsed in the film's opening sequence that shows Susy walking through the airport, and she also provides the gravelly voice of Helena Markos.
The trivia items below may give away important plot points.
The film's finale was inspired by a dream that co-writer Daria Nicolodi once had. In the dream she said she had encountered an invisible witch and, most bizarrely, there was a panther in the room with her that suddenly exploded. The dream was written into the film, but it's a porcelain panther that explode.
Stefania Casini had a difficult time shooting her death scene. Though the "barbed wire" that she falls into was fake, the coils of wire still got wrapped around her tightly and pinched her skin painfully.
Jessica Harper said in interviews that the most frightening scene in the film for her was the grand finale where everything explodes and shatters around her as she flees the academy. Harper said that the rigged explosions were quite unnerving as they went off close to her.