Alan Rickman, famous to many for his role as Snape in the Harry Potter films, knew a lot about the story arc of his character thanks to tips he got from author JK Rowling while filming. Following his death, The Independent reported that he previously revealed he had a little piece of information about Snape from Rowling that he had never disclosed, and never would. Rickman called Rowling's tip not crucial to the story in any way, nor a plot point, but "crucial to me as a piece of information that made me travel down that road rather than that one or that one or that one."
Rowling has revealed it herself, at the behest of a fan on Twitter. "Will you tell us the piece of information that you told Alan Rickman about Severus Snape?" Sarah asked Rowling. And her answer is full of meaning for Potter fans.
The word in question is found in the final Potter book, when it is revealed that Snape was always in love with Harry's mother, Lily. It explains so much of Snape's motivation, both in despising Harry's father and in protecting Harry. If her answer makes you tear up, you are not alone.