He was behind her the entire time waiting for the exact correct time to make his move. His breathing was practically non-existent. He was at that moment nothing more than a shadow, a form with no substance. And from his position behind her he observed her. When she moved forward, he moved forward with her. It was only when he could sense that she would sprint off soon that he returned to his physical form.
And in his physical form he still had the knife right at her neck. "Boo!"