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 for the playwright and the same was more explicitly true in "Precious Sons" (1986), George Furth's stage memoir set on Chicago's South Side in the summer of 1949. The then 14-year-old Rapp won raves for his extraordinary performance as the sensitive, dreamy youth who must choose between the theater and a conventional education and the concomitant competing wishes of his mother (Judith Ivey) and father (Ed Harris).
In films, the young actor made his debut as a lad with a crush on an older woman, teen-aged baby-sitter Elisabeth Shue, in the enjoyable "Adventures in Babysitting" (1987), the directorial debut of Chris Columbus. Rapp was also McGoo, an obnoxious, bespectacled anti-Semite in the 50s-set "social problem" drama "School Ties" (1992) and far more appealing as a high school outsider who gets lucky in Richard Linklater's "Dazed and Confused" (1993). He barely registered, though, amid the noisy effects in a tiny role as part of Cary Elwes' scientific team of tornado chasers in Jan De Bont's summer blockbuster "Twister" (1996).
Born in Chicago, Rapp was raised in nearby Joliet, IL where he developed a love for performing. He began appearing in musicals at age six and made his professional debut at nine in a road tour production of "Evita". As a child performer, he toured in "The King and I" with Yul Brynner and, by age ten, was briefly on Broadway in a musical entitled "The Little Prince and the Aviator" (although the show never formally opened). Rapp originated the role of Ben, the son of affluent, gullible parents in the Broadway production of John Guare's "Six Degrees of Separation" and reprised it in Fred Schepisi's 1993 film version. Rapp has been openly and vocally gay (or, as he prefers, "queer") in his professional life since 1992, proclaiming that the revelation has not hurt his career in the least. Back on stage, he was Charlie Brown in the 1999 revival of "You're a Good Man, Charlie Brown," and he played the title role in The Commonwealth Shakespeare's production of Henry V in Boston in the summer of 2002.
More screen roles ensued as well: Rapp had a the lead role in the indie drama "David Seaching" (1997), playing a young gay man trying to find himself, and in 2000 he appeared in the amusing teen comedy "Road Trip". Rapp embarked on a recording career with the release of his debut album, Look Around in 2000, the same year he portrayed '60s pop songwriter-producer Van Dyke Parks, who collaborated with Brian Wilson, in the ABC telepic "The Beach Boys: An American Family." He also appeared as one of Russell Crowe's colleagues in the acclaimed film "A Beautiful Mind" (2001). Rapp then reunited with director Columbus for the big screen adaptation of "Rent" (2005) and reprised his role as Mark, though the film did not completely deliver the pathos or majesty of the live version.