William Hartnell was a well-known British film actor in the 40s and 50s. Famous enough to have his own fan club, for instance. His breakout role was playing Sergeant Fletcher in the war film "The Way Ahead", which is (rightly) an acclaimed performance. It did though serve to typecast him a bit, and he often ended up with gruff NCO type parts. To a certain extent, he was parodying that image when he appeared in Carry On Sergeant, and also the tv sitcom The Army Game - it's probably worth noting that he is effectively the straight man in these productions though. Other notable films he's in include Yangtse Incident, Brighton Rock, Hell Drivers, The Mouse That Roared and This Sporting Life. (It was this latter that made Verity Lambert think he might be a good choice for the Doctor.) Getting him for Doctor Who would have been seen as a big coup in those days. He's one of my favourite actors of all time, even regardless of his role in Doctor Who.