This moment occurs quite regularly in Audrey Hepburn movies, notably in Funny Face, Breakfast at Tiffany's, and My Fair Lady. It becomes all the more curious with Hepburn, because all these films thematize Hepburn's any-face-whomever. Barthes places Hepburn at the point in cinematic history when the face begins its slow dissolve into the plebeian, the end point of an evolution that starts with the face of Garbo. He says that even though Garbo's face offers to audiences the "Platonic Idea of the human creature" (Mythologies, 56), it already had that existential quality that distinguishes face from mask, flesh from divine, woman from idea that would flourish in Hepburn.
In the films of Hepburn, this any-face-whomever is found in what Deleuze calls any-space-whatever; she is star as objet-trouve. That is, she is picked out of the crowd, or rather, it is her "out of the crowd" nature that is recognized by a visionary male as a new form of beauty. You would expect that when the protagonist goes running after her through the city, that he is bound to find any number of similar beauties. There are plentiful bohemian booksellers, nations of Lula Mays, entire classes of Eliza Doolittles. The crowd, however, has turned; it is no longer faceless, opening up possibility, the future. Rather the crowd acquires the face of the beloved, the past, and stops all possible action.