André Morell had previously been approached to appear in the second Quatermass serial, Quatermass II (1955) after Reginald Tate's sudden death. The part eventually went to John Robinson.
What's in behind here? Anything dangerous?
Prof. Quatermass: Probably not.
Sladden: Ah, it's all right, I'm insured.
Sladden: Are you insured sir? It's a good thing to be insured, at least it cheers you up.
Calendars seen on sets throughout the serial always show the episode's actual live transmission date. Whilst providing a uniquely contemporary feel for BBC viewers of the original 1958/9 screenings, it make no sense within the internal narrative.
All commercial releases prior to the 2004 DVD box set are an edited omnibus version. Several scenes written to allow actors to move between sets during the live broadcast were removed, along with the opening and closing titles.