Was the casting of Alan Rickman as Snape based on the full knowledge of the future of that character? points out that J. K. Rowling gave some information to the actors of Hagrid and Professor Snape about the plots of future books of the Harry Potter series, in order that they can act their roles better. Quoting the interview from 2001
Lizo: It's been reported that you talked about the "back story" of some of the characters with some of the actors. Is that true?
JK: I'm very secretive about these things, but yes I did. I gave certain people certain information. I gave Robbie Coltrane some background on Hagrid, which I think he used superbly, because I think he's an absolutely fantastic Hagrid.
I also gave Alan Rickman a little bit on Snape.
To which people did Rowling give such secret information about the plot of books before they were published? If it's known, I'd also like to hear when (before which book) they spoke with them, the purpose why they chose that particular person, and what she told them. I'm interested only in information given in private to very few individuals, not information in public interviews or similar.