Rita Hayworth was to appear in this movie, but left after a short time on the set. She was replaced by Kim Novak.
Dr. Tremayne (Donald Pleasence) converses with Nicholas (Jack Hawkins, who was dubbed with Charles Gray). So one Ernst Stavro Blofeld (Pleasence, who played the Bond villain in You Only Live Twice (1967) (1967)) speaks to another (Gray having played the same Bond villain in Diamonds Are Forever (1971) (1971)).