Before all the Harry Potter books were published and we knew Ron and Hermione would have a son named Hugo, there was always a question in the air about Snape's true allegiances.
But because J.K. Rowling didn't want to leave Alan Rickman completely in the dark about his heroically loyal character, she told him what he needed to know so he could stay ~true~ to Snape.
She gave me one tiny, little, left of field piece of information that helped me think that he was more complicated and that the story was not going to be as straight down the line as everybody thought. If you remember when I did the first film she'd only written three or four books, so nobody knew where it was really going except her. And its was important for her that I know something, but she only gave me a tiny piece of information which helped me think it was a more ambiguous route.