When Morgan Neville signed on to direct a documentary that puts backup singers front and center, he anticipated a challenge. ''I'm making a film about people who are by definition not stars,'' he says. But Neville's subjects including soul singers Merry Clayton, Darlene Love, Lisa Fischer, and The Voice contestant Judith Hill clearly enjoy the spotlight, whether they're recalling bitter memories of their flirtations with headline stardom or showing off their incredible vocals. ''Being a backup singer means being able to sing on a dime,'' says Neville. ''Music is oozing out of their every pore.'' He gathered Bruce Springsteen, Mick Jagger, and Sting to pay tribute to their vocal supporters. In this movie, though, the rock stars fade into the background.