

Either way, she saw an opportunity while Erik was busy being vulnerable with his feelings and his guard was down.

In addition, I think she is genuinely curious what's behind the mask - she got a glimpse earlier, but never really got to look at his face. Which leads to the mask reveal! I think Christine's motivations are twofold: first and foremost, I think it's a Hail-Mary attempt to get people to help, by showing them clearly what she has known for the past several minutes. Meanwhile, Christine is going, "WTF, why has no one else figured out what's going on, why isn't anyone helping?" She's looking for 1. This leads to the reprise of All I Ask Of You - Erik knows that, as far as Don Juan goes, his jig is up, Christine knows it's him, it's time for him to make one more (public) appeal for her love. She goes from playing her part just fine, to "oh shit I'm stuck onstage with the murderous stalker Phantom." She tries to get away, at least to put a stage's-worth of distance between them, but Erik is persistent and comes after her.

THAT she's not used to! And so that is what "wakes her up" from her on-stage trance. That is, of course, until he gets physically close to her. That's how she practices, with his voice, isn't it? So hearing Erik instead of Piangi doesn't ring any alarm bells for her, because she's come to expect Erik's voice in unexpected times and places. My headcannon answer is that she's so used to hearing his voice everywhere - in her dreams, while she sings, whether it's there or not (he is, after all, "insider her mind") - that it seems so natural to her, that of course she'd hallucinate his voice singing opposite her on stage. My question has always been, why wouldn't Christine, who for so long only knew Erik from his voice as the Angel of Music, realize immediately at the first line that he sings onstage with her that he was not in fact Piangi? Why does she go through most of the song seeming absolutely ok with this, and it takes Erik getting physically close to her for her to realize that it's him? I'm going to take the opportunity to ramble about them now! Oh man, I have a lot of thoughts about the Point of No Return, and that unmasking.
