Trying to cast the lead role of Mel Coplin, an adoptee searching for his biological parents in the wake of his own son's birth in the comedy "Flirting With Disaster" (1996), writer-director David O. Russell knew what he wanted: "a young Dustin Hoffman type, who was kind of urban, kind of smart and ethnic." Ben Stiller, the only son of the venerable husband-and-wife comedy team of Jerry Stiller and Anne Meara, convinced Russell that he could fill the bill. Increasingly busy before and behind the camera, the curly-haired, quirkily handsome actor-writer-director seemed well poised to become the