In The Mouse That Roared (1959), Peter Sellers played three roles: the Grand Duchess Gloriana of the tiny Duchy of Fenwick, its Prime Minister, Rupert Mountjoy, and the reluctant soldier Tully Bascombe, who leads an invasion of the United States. The comedy turns on a plan that goes wrong in the best possible way, and Sellers carries it in three very different skins.
As Gloriana he is regal and amused, as Mountjoy he is scheming and pompous, and as Tully he is ordinary, decent and out of his depth. The film, based on Leonard Wibberley’s novel, is a gentle satire on war and diplomacy, and it works because Sellers never lets the triple trick become the whole point. Each character has a reason to be there, and the audience can follow all three without losing the joke.