Recall the image of the lover, who, having lost his beloved, wanders the streets, despondent.
He (and it seems always a "he") suddenly sees the back of the head of the one for whom he searches, approaches her, and turns her around, only to realize it is merely a woman with a similar coif.
There is a complication of vision here: when he sees the back of the woman's head, he believes he is encountering her straight on. Yet when he finally turns her on her heels, his gaze goes blank--as if he were just presented with someone's anonymous backside. It's like a nightmare where there is no control of orientation: turning towards becomes a turning away and vice-versa.
But what he turns in this instance is not an anonymous woman, but the whole world. Seeing the back of her head, we are in the world of the musical in which everything is possible (and these moments indeed happen most often in musicals). But when he turns her, he sees something more or less like neorealism. The otherness of the human face erupts--infinite, empty space (what Lacan calls the extimate).