This is the turning point in the series where it begins to get a little darker and away from the kid friendly confines of the first two novels. The book as usual does a great job at exploring some of the loop holes in the movie. For example, it's never made 100% clear to me in the movie how exactly Sirius Black escaped Azkaban prison (but they do briefly tell you in the book). Also, some more information about how exactly Snape knew Black, Lupin, Ron, Hermione, Harry, etc. were in the Shrieking Shack and how to find them. Just little things like that that I was thinking to myself as I watched the movie again recently.
