Blond, wiry and intense, this likable twentysomething actor has proven his mettle on the New York stage in a variety of genres as a child performer, juvenile and young adult. Rapp has also established a steady career as a character player in films. His greatest notoriety to date has come on the Off-Broadway and Broadway stage, acting and singing the part of Mark Cohen, the uptight aspiring filmmaker who "narrates" much of "Rent" (1996), the late Jonathan Larson's musical phenomenon, for which he shared an OBIE Award with the ensemble cast. His character was generally perceived as a surrogate