J.K. Rowling reveals the secret that shaped Alan Rickman's role in the Harry Potter films

The role of Severus Snape in the Harry Potter films became one of the most memorable and celebrated parts in the long career of Alan Rickman, who died of cancer last week at 69. Rickman said many times that J.K. Rowling, the author of the Harry Potter novels, once gave him a secret piece of information that was crucial to his portrayal of Snape, and Rowling revealed that secret on Twitter today.

One of the many difficulties in making the Harry Potter films was that the story was still unfolding during their production — the seventh and final book, Harry Potter and the Deathly Hallows, was not published until July 2007, the same month in which the fifth movie premiered. When production began on the first film in September 2000, only the first four novels had been released.

[Warning: The following contains spoilers for the Harry Potter series.]

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Harry Potter fans will know that Snape, the potions master at Hogwarts, is perhaps the most complex character in the entire series. There's much more to Snape than meets the eye, and his true nature is not revealed until late in Harry Potter and the Deathly Hallows.

Rickman said in an interview with RTÉ in 2011 that he had a phone conversation with Rowling prior to the start of production on the first Harry Potter film. He said that while Rowling didn't tell him what Snape's fate would be, she did reveal something important that led him to imbue the character with nuance.

"She gave me one little piece of information, which I always said I would never share with anybody and never have, and never will," Rickman told RTÉ. "It wasn't a plot point, or crucial in any tangible way, but it was 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."

As he promised, Rickman took that secret with him to the grave. But a fan asked Rowling yesterday if she would reveal what she told Rickman, and she obliged today on Twitter.

I told Alan what lies behind the word 'always'. https://t.co/NHTJ5J6kxb

— J.K. Rowling (@jk_rowling) January 18, 2016

If you've got a copy of Deathly Hallows lying around, flip to chapter 33 — specifically, to page 687 (in the original hardcover edition, anyway) — to help yourself remember what Rowling meant. Failing that, you can watch the relevant clip from Harry Potter and the Deathly Hallows – Part 2 below. Either way, you may want to find a box of tissues before you do.

"Always" refers to a pivotal moment in the final book in which Harry is going through Snape's memories — memories that Snape left for Harry while dying. In one sequence, Snape surprises Hogwarts headmaster Albus Dumbledore by revealing how much he loved Harry's mother, Lily Potter, and that he would do anything to protect Harry for that reason, despite his hatred of Harry's father, James Potter.