Professor Quatermass: was never played by the same actor. Reginald Tate played him for the first series, but died shortly before transmission of the second series, and was hurriedly replaced by John Robinson. In Quatermass & the Pit he's played by Andre Morrell, and by Sir John Mills in a 1979 version.
Films: Hammer made films of all three plays. Author Nigel Kneale quite likes the last film, but loathes the others. Rumours persist of big budget remakes (though the premise of The Astronaut's Wife bears a lot in common with The Quatermass Experiment).
Live transmission: Let's just repeat that. These shows were made and shown live. Quatermass II saw the relative innovation of film recordings during transmission - this meant that the programme could be repeated without the cast having to turn up and do it all over again, which happened quite frequently.
Received Pronounciation. This was still the era when everyone on the BBC spoke BBC English. Not only is Monica Grey as the Professor's daughter frightfully posh, but so too are all the RP cockney and Welsh characters.
Wilfred Brambell: (Steptoe and Son) and Roger Delgado (The Master in Doctor Who) are among the many familiar faces.
Special Effects: were done by pioneers Jack Kline and Bernard Wilkie. They achieved an exploding alien dome by blowing up a model full of milk in a fishtank. They also used black rubber gloves to portray the evil alien horde.
Zombie marks: In early episodes its quite hard to see just who has been taken over by a Zombie. In later episodes the make-up gets rather more dramatic (shudder).
Talking to yourself: Shortly before filming Monica Grey suddenly acquired another character's lines as well as her own. Frequently, she appears to answer her own questions.
Tunes: Dramatic stock music was played live to increase dramatic tension. It sometimes arrived a little early. With startling effect.
Spoiler tactics: Quatermass II was broadcast just after the start of ITV - was it another attempt to steal the new network's audience?
Hammerweb - Information on The Hammer House of Horror film of Quatermass II.
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