Director Dario Argento's original idea was the ballet school would accommodate young girls not older 12. However the studio and producer (his father) denied his request because a film this violent involving children would be surely banned. Dario Argento raised the age limit of the girls to 20 but didn't rewrite the script, hence the naivety of the characters and occasionally childlike dialogue. He also put all the doorknobs at about the same height as the actress' heads, so they will have to raise their arms in order to open the doors, just like children.
It is often incorrectly assumed that, to achieve the rich color palette, the film was shot using the outdated 3-strip Technicolor process. This is untrue. No film made after the mid-1950s was shot using this method. The film was instead shot on normal Eastman Color Kodak stock, then printed using the 3-strip Technicolor process, utilizing one of the last remaining machines. This issue has been confused somewhat by the fact that, on the 25th anniversary documentary featured in the 3-disc DVD set, a discussion of the printing process by cinematographer Luciano Tovoli was incorrectly followed by a diagram showing a 3-strip camera.
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. Daria 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 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.
The films finale was inspired by a dream that co-writer Daria Nicolodi once had. In the dream Nicolodi 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, only in the film it's a porcelain panther that explodes - rather than the real panther that appeared in Nicolodi's dream.
Star 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 where quite unnerving as they were placed close to her on the sets.
According to star Jessica Harper since the film was going to be dubbed after principle 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 stage hand hammering away on another set in the studio.
In an interview with star Jessica Harper, she said that many of the actors on set spoke different languages during shooting. According to Harper most either spoke Italian or German and it would make communicating difficult at times. However since the film would be dubbed for American release it was deemed not to be an issue during filming.
The trivia item below may give away important plot points.
Star '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.