Actor Ronny Cox in this film reprized his stage role of Sergeant Boford Miles from the 1970 New York theatre production.
The film in the English language has had a number of titles. These include: The Mind Snatchers, The Happiness Cage, Mindsnatcher, Brain Control, Mind Snatcher, The Demon Within, Mind Snatchers, and The Mind Snatcher.
''First major movie role'' of actor Christopher Walken according to a UK DVD release box cover.
In the original stage play, Dr. Frederick (Joss Ackland) was named Dr. Freytag. In this 1970 New York stage production, the character was portrayed by actor Henderson Forsythe.
Actress Bette Henritze reprized her 1970 New York stage production role in this film version however her character's name was slightly different in the two adaptations. She was called Anna Ames in the theatre play then re-named Anna Kraus for this feature film production.