When Alan Rickman started playing Professor Severus Snape, the end of the Harry Potter series was not yet written—but JK Rowling already had Snape's story arc figured out, and she hinted to Rickman that (spoiler alert!) there was more to Snape than met the eye so that he'd understand the character's motivation and know how to play him. Rickman had hinted at that fact in interviews, Mashable reports, but he never revealed exactly what Rowling told him—but now Rowling finally has, thanks to a fan on Twitter who asked her, "Will you tell us the piece of information that you told Alan Rickman about Severus Snape? Or will that forever be a secret?"
As the Independent explains, Rickman once said that Rowling didn't reveal the entire ending of the series to him, but rather gave him "one little piece of information, which I always said I would never share with anybody and never have, and never will," and that helped him "travel down that road rather than that one or that one or that one." Rowling's response to the aforementioned fan explains what that "one little piece" was: "I told Alan what lies behind the word 'always,'" she wrote. Spoiler alert part 2: She's referring to the fact that, as Snape reveals to Dumbledore toward the end of the series, he's "always" loved Harry's mother, and thus felt compelled to protect Harry. (Watch Snape's scenes in chronological order and all will become clear.)