Off-beat, young American supporting player whose film roles have been somewhat less challenging than his stage work, which has encompassed plays by Shepard, Brecht, Sartre, Beckett and others. Nonetheless, Diehl's vaguely dissipated, all-American features and toothy grin have brought an edge to almost all his performances, particularly as a cheerfully brutal villain in "Mo' Money" (1992), Damon Wayans' breakthrough comedy/action vehicle. He again exhibited a flair for sleaze as an obnoxious young cop who needles the retiring Robert Duvall in Joel Schumacher's "Falling Down" (1993)....