Joaquin Phoenix had already played a music star once — Johnny Cash in the 2005 biopic Walk the Line. What was left but to become one himself? That's as good of an explanation as any for the reason Phoenix abruptly decided to quit acting in 2008, grow a bedraggled beard and focus on his not-so-burgeoning rap career. The announcement was so strange — and the rapping so bad — that it led many to question whether this was some sort of bizarre piece of performance art. Phoenix has insisted he's serious, though a disastrous interview with David Letterman did little to dispel the disbelief.