I would just add that for recording some musicians/vocalists have their own studios and for a set fee will, on a work for hire basis, lay down parts to order. You send them a click track or guide and they send you back their completed part.

For live gigs it is a recipe for disaster hiring someone in at short notice who you do not know or does not know your music. Even doing covers when they know the songs can be a disaster. Proper rehearsals are necessary to work out things like arrangements, key, harmonies and how the song will finish.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055