According to a 1969 issue of Photoplay, Pia Degermark was the first choice for the title role. Other contenders included Connie Kreski and Sydne Rome.
Director Christian Marquand (with camera crew) appears briefly through reflection in mirror-walls, in the final scene, filmed outside, with all actors together.
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 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.
Loose adaptation of Voltaire's "Candide" reconfigured for the permissive 1960s.
Pamela Des Barres writes in her landmark book "I'm With the Band" that she, too, was considered for the title role.
Playboy Playmate DeDe Lind was offered the lead role, but turned it down.
The film was entirely financed thanks to Marlon Brando's participation. Marquand and Brando knew each other for a long time and were good friends. Brando agreed to do the film as a favor to his friend. 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.
The sequence of the nurse assisting Dr. Krankheit put on his latex operating gloves is a clever reverse film of pulling the gloves off.