Christine sings badly when she sees Raoul

[igenlode]

During the doomed performance of "Faust", immediately before the toad episode, there's a moment when Christine, singing the role of Marguerite's young suitor Siebel, looks up and notices Raoul sitting in the Chagny box. Her voice immediately loses its assurance and crystalline clarity and becomes nervous and dull (and Raoul tries to hide his tears by burying his face in his hands).

Why does seeing Raoul have this effect? The previous time they had seen each other was when Christine was nursing Raoul out of his fit of hypothermia at Perros-Guirec, after which she vanishes without saying goodbye and sends him a letter to say that they can never meet again.

Is it the sight of Raoul's distress that affects Christine's singing, or is it the spectacle of Christine's botched performance that causes Raoul's distress? I think I've seen this scene cited in the past as evidence that Erik inspires Christine to perform to greater heights, while Raoul actually sabotages her by his mere presence :-p