The death of the actor from cancer was announced today. He played a lot of villains in his career, but he was a very versatile actor and could do deft comedy, as in the film Galaxy Quest (2000) where he played a Shakespearean trained actor frustrated by being mainly identified with a popular Star Trek like TV series character.
But he will undoubtedly be remembered most for his role as Severus Snape in the Harry Potter franchise. In many ways his was the most central and interesting character in both the books and the films because one did not know what his motives were and whether he was as truly evil as he seemed. Here he is talking about how he played the part and how, since only three books had been written when he was cast, he dealt with playing a role where did not know the true nature of the character.