In one sequence Charles Aznavour (as the hunchback) escapes the police by crawling on the mansion ceiling and jumping down through a window made of water. This is a visual reference to surrealist Jean Cocteau's The Blood of a Poet (1932) where the character jumps into a mirror made of water after crawling over a hallway of doors (the effect was achieved by constructing the "wall" and "ceiling" on the studio floor and shooting the scene from above). Even the sound effect at that moment, a shouted "No!", is in both films.
The film was entirely financed thanks to Marlon Brando's participation. Director Christian Marquand and Brando had known each other for a long time and were good friends. Brando agreed to do the film as a favor to his friend. Marquand had helped with negotiations with French Premier Georges Pompidou in his purchase of the Tahitian island of Tetiaroa. He didn't particularly like the script but he had fun shooting his sequence. Then other stars agreed to play a part in the movie and it was even easier to convince the producers.