Concertgoers at L.A.'s legendary Roxy Theater on Monday night were in for quite a surprise when Gwyneth Paltrow took the stage to sing backup vocals for singer and songwriter Holly Williams. The Goop founder crooned behind the mic as one audience member tweeted, "'Just Gwyneth singing w @hollywilliams1 at the roxy. NBD." The blond beauty and the country star opened for Jason Isbell in front of a packed house.