The paradox is that when the head is turned, the director can have more control over the performance, because s/he can freely dub in dialogue (or erase dialogue) without having to sync it up perfectly to the lips of the actor. In Vertigo, this technique is used to great advantage, the non-sync dialogue a patter of dreamlike phrases, as if they were a product of the very projection process that Scottie is caught up in, which is the psychic process of the director as well (this film is often called autobiographical). It's as if these floating, non-sync snippets come straight from the director's unconscious, animating the b-roll like one charms a lock of hair. The unlock of hair, the lock of her, interlocutors.