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.