Hartnell continued to play comic characters until he was cast in the robust role of Sergeant Ned Fletcher in "The Way Ahead" (1944). From then on his career was defined by playing mainly policemen, soldiers and thugs. This typecasting bothered him, for even when he was cast in comedies he found he invariably played the "heavy". In 1958 he played the sergeant in the first Carry On comedy film, "Carry On Sergeant". He appeared as Will Buckley, another military character, in the film "The Mouse That Roared" (1959), which starred Peter Sellers, and he played a town councillor in the Boulting brothers' film "Heavens Above!" (1963), again with Sellers.