In an interview yesterday on NPR they said that Chris Columbus, who directed the first couple of Harry Potter movies, remembered Die Hard and wanted Rickman, who wasn't convinced that the role was right for him. Apparently J.K. Rowling herself took Rickman aside and told him about how the character was going to develop in the planned but as yet unwritten volumes, and his importance to protecting Harry through the sequence of books. That's when Rickman took the role.