Guest judges? Sure, other shows have 'em, but mentors? Not like these. The last go-around, the four coaches enlisted the help of their famous friends in coaching the contestants: Monica, Reba McIntire, Sia and Adam Jackson. This year, the list includes Alanis Morrissette, Ne-Yo, Miranda Lambert and Lionel Richie. The guests bring extra star power, as well as insight in guiding the contestants, some of whom have little traditional training. Like the judges, these are people with active music careers — i.e., people who actually know what they're talking about.