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The cathedral in the climactic scenes was given the fictitious name of Minster Cathedral (even though it was intended to be Westminster Abbey) because Jack Gold did not want anyone who was familiar with the architecture of the Abbey to say "but that's not Westminster Abbey", given that the scenes were actually filmed at Bristol Cathedral.
Jack Gold did not want Richard Burton and instead suggested Nicol Williamson for the lead role. The producers told him it would be easier to get funding with Burton, who had just made his "comeback" movie Equus (1977).
Filmed hurriedly in the early summer of 1977 so Richard Burton could begin filming The Wild Geese (1978) in South Africa in the late summer.
Some of the movie posters featured a text preamble that read: "TELEKINESIS: A mental force that enables this man to move objects and control events. Science cannot explain the awesome power of the mind. And nothing can control it. Richard Burton has THE MEDUSA TOUCH".