The scene in which Gillie is reenacting the murder inside the apartment for the police was ad-libbed by Hayley Mills. Sir John Mills and Director J. Lee Thompson told her the basic plot for the scene and instructed her to just make it up.
The role of Gillie was originally written for a boy. When Director J. Lee Thompson went to see Sir John Mills, he noticed that tomboy Hayley Mills had her hair like a boy, and he offered her the part. for which she won a Silver Bear at the Berlin Film Festival.
Hayley Mills admitted to being strongly infatuated with her handsome male co-star, Horst Buchholz, even though he was always putting her off her lines by goofing around behind the camera.