Boris Karloff came onto the production because he was under contract with American International Pictures who were interested in doing another picture with Mario Bava.
Director Mario Bava's father, Eugenio Bava, created and sculpted the head for the ghost in 'The Drop of Water' as well as the served head for the segment 'The Wurdalak.'
The films Italian title is "I Tre volti della paura", which means "The Three Faces of Fear."
The films title was a homage to 'Black Sunday' the previous film that Mario Bava had made for AIP. The success of that picture made the distributors select a title that was similar to that of the earlier Bava hit.
The segment 'The Telephone' was the first Italian thriller to be shot in color.
There were additional scenes filmed with Boris Karloff introducing the segments, however AIP decided they were unnecessary and cut them from the film. Karloff would later say these introductions were some of the most fun he'd ever had on a film set.
This film served as inspiration for the naming of the heavy metal band Black Sabbath. In 1968, the band (then called Earth) was playing a small club in Birmingham, England. Across the street was a movie theater showing the film Black Sabbath. The band noticed that more people were in line to see the movie than were to see the band. Realizing that "horror sells tickets" they decided to change the band's name to Black Sabbath.
This is the only film in which horror icon Boris Karloff plays a vampire.