Multiple sources state Ken Wiederhorn was the original director, and was replaced mid-production by Freddie Francis. However, Wiederhorn claimed in an interview that this was a misconception, that he had never been on-set and had never seen the final film. According to a May 1988 interview with the magazine Cinefantastique, Wiederhorn was initially attached to direct, but dropped out after financing fell through, and directed Il ritorno dei morti viventi 2 (1988) instead.
Jenny Agutter replaced Lucy Guttridge.
Michael Moriarty replaced Roger Daltrey.
Director Freddie Francis took his name off of the film because he was dissatisfied with the quality of the special effects. He disliked the experience of directing it so much he left film directing behind and returned to cinematography full-time. His only subsequent directing credit was an episode of I racconti della cripta (1989) (Last Respects (1996)).
The building used for the outside shots and the falling window cleaner is on Gran Via de Carles III, 94, 08028 Barcelona, Spain.