Actually, the broad strokes of this film are true. Brady did make his fortune as a salesman for railroad equipment, wore loud diamond jewelry, had an enormous appetite, and he did know Lillian Russell for years. It's the individual scenes that were created by Preston Sturges for dramatic license. If the Gilded Age had not produced such a character, Preston Sturges would have invented him.
Brady falls in love with a southern belle from Charleston who marries somebody else and then years later he meets another woman, Jane Matthews, who looks just like her, yet the two women are not related. He falls for her only because she looks like the first love, but again, her affection is returned but not her love. The theme of the film being that Brady was a man with tremendous financial success but who never found love that was reciprocated.
Edward Arnold didn't get that many leading roles, and this was one of them that he seemed born to play. His Diamond Jim is bigger than life and a generous soul who just happens to also be a great salesman. Also note William Demarest as a waiter. He ended up being a staple in Sturges' films.
