Anthony Rapp, who’s famous for being the first Mark in “Rent,” had some great support at his Wednesday-night performance of “Without You,” his one-man show, which runs at the Modern Theatre through Sunday. Joining him backstage that night were indie film star Parker Posey, who has been in town for the Provincetown International Film Festival and the Boston premiere of “The Love Guide”; Actors and Kingston residents Chris Cooper and Marianne Leone Cooper, and Rapp’s “Without You” director, Steve Maler.