New Alan Rickman interview-

There is a longstanding story that Rowling let Rickman know early on that Snape was not the bad guy or Voldemort henchman in waiting many expected him to be. Before I could even finish my question about this anecdote Rickman jumped in to clarify.

"Not true. I don't know who thinks that is true, but it's not true," Rickman says. "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."