The Devil Rides Out is one of those movies, like Casablanca, that I can watch again and again. I wish Hammer had continued a franchise with Christopher Lee as the occult investigator, Duc de Richleau. Terence Fisher and Richard Matheson (screenplay) did a marvelous job with Dennis Wheatley’s rambling book, and the depiction of the more nefarious theme of Satanism (not the fun-loving Sabrina-esque one we’ve come to love these days), is still effective though not quite as shocking. Wheatley had written a few books around Duc de Rickleau and Lee, after meeting the author at a lecture, urged Hammer to lens one of his books. Lee is noted as saying this was his favorite Hammer movie (cited from IMDb). Considering he played the good guy fighting evil, for a change, I can see why.
Executive producer Anthony Hinds was inexplicably depressed by the rushes, and made a special request to composer James Bernard to detract from the film’s shortcomings. While Bernard was working on his score, hinds hired Patrick Allen to re-voice co-star Leon Greene, inadvertently compromising the film even more. A pivotal special effects sequence, a close-up of the Angle of Death, was left unfinished. ( The Hammer Vault, Treasures From the Archive of Hammer Films by Marcus Hearn)
Nevertheless, it would seem that Hinds was not the only person who had reservations about Greene’s acting abilities. Commented Christopher Lee in a letter to his fan club shortly after the completion of shooting, “What I have seen of the film, with the possible exception of one member of the cast, promises to be surprisingly good.” Ouch! (Hammer Complete: The Films, the Personnel, the Company by Howard Maxford)