The character played by Rosenda Monteros is referred to in some reference guides as "Conchita". Actually, she's called "Pili" in the original English version and "Paula" in the Spanish dubbing.
During production, producer Harry Alan Towers stopped paying his actors' salaries. While the rest of the cast kept working, Celeste Yarnall walked off the picture. Instead of paying her, Towers rewrote the middle of the script to have another female character played by Rosenda Monteros pretend to be Eve for a long stretch of the film until Yarnall returned.
Christopher Lee was upset during filming that he was cast to play Eve's grandfather because he felt he would have been much better as the leading man and her love interest than Robert Walker was. He also hated being made up to look old.
Discouraged that her career hadn't taken off, Celeste Yarnall headed with her husband Sheldon Silverstein to the 1967 Cannes Film Festival, hoping that she would wow some producer. She did. Producer Harry Alan Towers, who was looking for a girl to play the role of Eve, spotted her strolling down the street. According to Yarnall, "He yelled and pointed. 'Stop that girl! That's my Eve!'"
The ending left the film open for on intended sequel that was never made because the film was a financial failure. Star Celeste Yarnall said she was relieved because she thought this film was "one of the worst movies of all time." She also never thought she was right for the role and had no idea why she was cast.