Monica Grey played Paula Quatermass; she was chosen by BBC management rather than the production team, as she was the wife of the BBC's head of radio drama, Val Gielgud. As Hugh Griffith also had problems with some of his technical dialogue, Grey learned his lines as well as her own, in case she needed to step in and assist him during the live performance. Dillon was played by John Stone; Stone too had a long career as a supporting actor in a range of British television series, and in 1956 had a small role in the film "X the Unknown", which Hammer Film Productions had intended as a sequel to their version of "The Quatermass Experiment", until Kneale denied them the rights to use the character.