Prior to becoming a national icon, he had primarily played military types (such as the eponymous Sergeant in Carry On, Sergeant) and sinister hoods - he turned in a memorably creepy performance in Brighton Rock, where he was overshadowed by Richard Attenborough's turn as Villain Protagonist Pinky. Hartnell also appeared with Peter Sellers in 1959 film The Mouse That Roared again as a sergeant.
The First Doctor's crochety old man persona was partly influenced by his own nature, particularly because his memory was beginning to fail him; his blustering diatribes at his companions often went off-script, which, in an era when Television was essentially live theatre performed for a camera, resulted in more than a few occasions where the other actors are visibly struggling not to corpse and improvising like crazy to get the scene back on track.
His sadly increasing infirmity led to the creative team finding a new way to continue the show; since the Doctor was an Alien, who was to say what he might or might not do? He might even turn into a completely different person... and thus was born Regeneration, by means of which a television series that started in 1963 is still airing in 2012. He reprised the role of the Doctor once more, in the serial The Three Doctors, filmed in 1972; with his health in serious decline, he had to appear only in pre-recorded inserts, and read all his dialogue from cue cards, but still managed to deliver a spiky putdown of Patrick Troughton (who he had personally suggested as a worthy successor) and Jon Pertwee; "So, you're my replacements, hmm? A Dandy and a Clown!"
In 1975, after a long illness, he died in his sleep; his death was reported on the BBC National news, illustrated - naturally enough - with a clip from his time as the Doctor.