The story goes that Merry Clayton was woken in the middle of the night and asked to sing backup for The Stones.  With little to no preparation, she laid down some of the best backing vocal work ever. Listen to it here.

 

This is a clip from "20 Feet from Stardom", a documentary about backup singers that is near the top of my list of movies to watch, soon to be featured in [Celluloid Hero].