Peter Cushing arrived in Spain for filming and immediately told producer Bernard Gordon that he could not do the picture, as he felt it was too soon after his wife's death. Christopher Lee convinced Cushing to stay on by reminiscing with him about the previous movies they'd worked on together, much to the relief of Gordon.
Filmed in December 1971, the first Christmas for Peter Cushing since the January 14 death of his beloved wife Helen. Sir Christopher Lee's family made it as warm an affair as possible for Cushing, who would grieve for his lost wife for the remainder of his life, often playing roles that mirrored his own sadness. Despite the freezing working conditions and "abominable" food, this movie provided one of the few co-starring roles where the two actors got to work in unison, rather than opposing one another, with Cushing getting the most amusing lines.
It has been said that Peter Cushing was so devastated by his wife's recent death by the time this movie was shot that he would suffer from night panic. In order to make his nights easier, so he would not be alone when the attacks happened, Christopher Lee used to sleep in his same bed.
Frequent airings on television throughout the 1970s and 1980s helped to gain this movie a devoted cult following amongst horror fans.
Most of this movie was shot without audio recording; the soundtracks and dialogue were all added in post-production. Sir Christopher Lee, Peter Cushing, and Telly Savalas provided their own voices for this movie's English-speaking version.