Alan Rickman and Jim Broadbent
In the Harry Potter movies, well-known actors play well-known book characters. This doesn’t mean that the actors are perfect fits for the stories the film is based on, though. The movies about Harry Potter are based on the books that J. K. Rowling wrote. Because Alan Rickman was so interesting as Snape, it was natural to think that Jim Broadbent’s casting as Professor Slughorn would confuse people who had seen him play that part. Jim Broadbent was chosen to play the role because of this.
Before they worked together in the sixth Harry Potter movie, Slughorn was one of Snape’s Hogwarts professors. Even though they work together in the film, this is the case. Both Slughorn and Snape made appearances in the movie. Even stranger, Rickman was three years older than Broadbent in real life, which makes this even more of a puzzle.