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.
Regarding daughter Hayley's acting abilities, John Mills said when they started filming "I simply couldn't believe what was happening. She looked as if she'd been born in front of a camera."
Regarding daughter Hayley's acting abilities, John Mills said when they started filming "I simply couldn't believe what was happening. She looked as if she'd been born in front of a camera."
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.
The role of Gillie was originally written for a boy, as written in the short story. 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. The 11-year-old was not interested in acting at the time but accepted the challenge anyway. Her father has admitted that once work started on the film he was bowled over by her natural skills. She would go on to be nominated and win numerous awards in the next five years, including a Silver Bear at the Berlin Film Festival for this role.