According to Sir Christopher Lee, this movie, dealing with an uncommon heat wave, was shot in the middle of winter. The actors and actresses not only had to be covered with glycerin to create the illusion of heavy sweat, but also suffered from wearing very light clothes in a freezing season.
Patrick Allen and Sarah Lawson play a married couple in this film. They were husband and wife in real life as well.
For a magazine interview about her experiences on making the film, Jane Merrow has said how Christopher Lee looked rather imposing and intimidating. On the other hand, she found Peter Cushing to be very charming and courteous.
The original script was ditched at the last minute as being unworkable.