When I first read Chapter 2, I thought, my god, isn't Snape a marvelous actor? And when I read Dumbledore pleading, I never thought once that it was for his life. I thought, This man knows he's going to die, he's pleading for Snape to finish it. When Harry was immobilized, that's what clinched it for me - Dumbledore wanted Harry to stay and watch, because he knew that ultimately, Harry would realise what Snape was doing, maybe not today, maybe not tomorrow, but eventually.