The movie began under the direction of Joseph Losey (working as Joseph Walton), exiled to England because of the Hollywood blacklist. However, when Dean Jagger arrived he refused to work with a director he thought of as a Communist sympathizer, and Losey was replaced by Leslie Norman before shooting began. Losey's departure was publicly attributed to "illness". It has also been reported that Losey simply didn't want to make the film and left the project, to be replaced by Norman, who also didn't want to make the film, but did anyway.
According to Jimmy Sangster, director Leslie Norman was not overly interested in making this film, but did anyway for the sake of work. Dean Jagger and other crew members, not to mention Hammer brass, all found Norman very difficult to work with, so much so that despite the film receiving some good notices, Hammer never worked with Norman again.
The film was originally intended to have been a sequel to another Hammer success, The Quatermass Xperiment (1955), but creator Nigel Kneale vetoed the use of his character(s) by another writer--hence Prof. Bernard Quatermass swiftly became Dr. Adam Royston.