According to actor Michael Murphy, director Saul Bass was very worried about British actress Lynne Frederick being able to sound suitably American for the film, since her character was supposed to be from Arizona. Bass made her run her lines over and over while he listened for any hint of a British accent.
Lynne Frederick had to wear a specially designed iron clad corset during filming to appear more flat-chested. Kendra, the character she played, was supposed to be about 16 years old, and the director wanted to minimize her voluptuous figure. Frederick was also encouraged to restrict her diet to chicken broth and black coffee.
Science Fiction writer Barry N. Malzberg wrote a novel based on Mayo Simon's original script. As a result, the ending of the novel differs from the film.
After working with actress Lynne Frederick in No Blade of Grass (1970) four years prior, Nigel Davenport personally recommended Frederick for the role of Kendra.
Kendra is barefoot throughout the film to emphasize her youth, childlike innocence, and country background.