I do not recall that distinction being made clear at all. I believe people freely talked about the new cast members in the unspoiled thread, after all many of them want to talk or discuss things about the upcoming season and certainly at the comic cons and the like they make big announcements of new cast members in front of non-book readers.
Now clearly there is a line. If you say "Stannis Baratheon is being cast and will be in season 2, he is the middle brother of Renly and Robert, you might remember he left King's Landing shortly before Ned got there", how does that spoil anything? Now if you instead said "Stannis Baratheon is being cast, boy I can't wait until he goes to sack King's Landing and kills his brother Renly", yeah.