Artist John Bolton painted the picture of the Shadmock used in the film before the role was even cast. He was genuinely amazed when the film's producers found an actor, James Laurenson, who actually physically resembled the image in the picture.
This is the only film, in a long career of horror and other roles, where Vincent Price played a vampire.
Milton Subotsky claimed he originally planned for the film to include all six of the six top horror film stars at that time in major roles in it - that is, Vincent Price, Peter Cushing, Christopher Lee, Klaus Kinski, Donald Pleasence, and John Carradine. Cushing, Lee and Kinski all turned it down, but Price, Pleasence and Carradine all appeared in the film.
Christopher Lee was approached to play the role of R. Chetwynd-Hayes in the film, which was ultimately played by John Carradine.