As an actor, with his domineering personality, bulky physique (he played rugby for Beckenham RFC First XV in the 1924–25 season alongside Johnnie Cradock who would become the partner of 1950s TV chef Fanny), and rich, booming voice, he was soon established as a major supporting player in British comedy films. His first leading role was as headmaster in the film "Vice Versa" (1948), written and directed by Peter Ustinov, who cast him partly because he'd been "a collaborator of my father's at Reuters." Justice was the demanding surgeon Sir Lancelot Spratt in the "Doctor" series of films of the 1950s and 1960s, beginning with "Doctor in the House" (1954), playing a role for which he is possibly best remembered. In his films he was sometimes credited as "Seumas Mòr na Feusag" (Scottish Gaelic, translation: Big James with the Beard), "James R. Justice", "James Robertson" or "James Robertson-Justice".