Late-night host David Letterman has done his share of bizarre celebrity guest interviews, but a February Late Show appearance by actor turned aspiring rapper Joaquin Phoenix was one for the books. The Walk the Line star ambled onstage in what might charitably be described as a catatonic stupor, sporting a tangled mane, bushy beard and dark shades. Seated, Phoenix offered clipped, monosyllabic responses to Letterman's questions, picked at his clothes and gazed aimlessly at his lap. At one point he even took his chewing gum out of his mouth and affixed it to the host's desk. As the segment drew to a close, Letterman hurled one of his greatest-ever quips: "Joaquin, I'm sorry you couldn't be here tonight."