I heard from my inside sources that he didn't want to sing for two reasons. Reason one is that he was tired after the weekend performances of "Wizard of Oz". He had done both the matinee and evening performance. Reason two was that he didn't know if he would be able to sing cause he gets very emotional when it comes to the show that changed his career and life. And as you can see he is already crying 5 seconds in.
Also the fact that Sarah Brightman "sang" was not part of the plan either. They had originally four Christines lined up as well who would sing different part of the song.
But Lloyd Webber managed to persuade her to sing. Wished he hadn't

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