Quatermass was a british rocket scientist in a pioneering series of early television science fiction on the BBC. There were originally three series (incidentally broadcast live to camera) with a forth added decades later by Thames Television.
The series was successful enough that all the original series were remade as movies. But only one actor played Quatermass more than once. Why were there so many changes?
The Broadcast and release dates with the actor playing Quatermass (sequence might not be quite right and the movies names were different in the USA)
The Quatermass Experiment (TV 1953) Reginald Tate
Quatermass 2 (TV 1955) John Robinson
The Quatermass Xperiment (movie 1955) Brian Donlevy
Quatermass II (movie 1957) Brian Donlevy
Quatermass and the Pit (TV 1958) Andre Morell
Quatermass and the Pit (movie 1967) Andrew Kier
For completeness (but less relevant due to the long delay before these were made):
The Quatermass Conclusion (TV series and shorter movie 1979) John Mills
The Quatermass Experiment (TV remake broadcast live 2005) Jason Flemyng