Updated on January 14, 2016: Alan Rickman has passed away, with Harry Potter's Severus Snape remaining one of the actor's iconic roles. This edit shows just how much heavy lifting it took to create a story we would only really understand once the series was complete.
He did this all without knowing exactly what that story would be.
"[Rowling] gave me one tiny, little, left of field piece of information that helped me think that he was more complicated and that the story was not going to be as straight down the line as everybody thought," he said in an interview. "If you remember when I did the first film she'd only written three or four books, so nobody knew where it was really going except her. And its was important for her that I know something, but she only gave me a tiny piece of information which helped me think it was a more ambiguous route."
Original story: Severus Snape is one of the most interesting characters in the Harry Potter saga, but his true motivations are hidden for the majority of the books and movies. Learning why he acts as he does, and why he's chosen his particular path, is fascinating the first time through the story, and it's even better when you re-read or watch knowing what's going on.
That being said, when you grab all of Snape's scenes and show them in chronological order you can really see the tragedy behind the character. This is an interesting way to revisit a story most of us know by heart, and it's even more heartbreaking when his path is made explicit.