The casting and the musical numbers are the strengths of The Big City, a maudlin little melodrama that would otherwise not be worth watching. Oh, City is certainly well-intentioned and carries a message of understanding, tolerance and cooperation that is entirely laudable. But it wraps that message up in a story that is trite, sticky, excessively sentimental and quite manipulative. There are individual scenes that work, but on the whole the screenplay is a liability. Fortunately, Margaret O'Brien, almost a teen-ager, is still young enough to make this mess work, aided immeasurably by the likes of Robert Preston, George Murphy and Danny Thomas. This three men and their little lady provide enough charisma and personality to make up for most of the script's failings; and what they can't overcome, Betty Garrett, in her film debut, can. Throw is some invaluable assistance from Edward Arnold and Karin Booth and bits by Connie Gilchrist and Butch Jenkins and City becomes surprisingly entertaining -- especially when the screenplay gets out of the way and lets the players engage in some nifty singing and stepping.