In any big spectacular, it's really difficult to have enough voices to cover all the vocal parts. To give the audience the complete experience they're expecting, there is some reinforcement, some playback that everybody's hearing. Sometimes it's background vocals, but sometimes it will be actually vocal tracks. It's so hard to ensure, with no safety net ... you're not gonna get another shot at it, you have to have stability. I think it's very naïve of a lot of people to think that when you see someone open their mouth, they're really singing.
