I know this article has been making the rounds (I saw it on reddit's front page last week), so many readers have presumably already seen it, but if you haven't, check out Emily Asher-Perrin's article entitled "Erased By Time and Blockbusters-the Cautionary Tale of Ron Weasley".
It's a good illustration of the difficulties that you face when turning a giant novel with a lot of character development into a two or three hour movie. Also, it shows how hard it is to balance three characters in a movie and give them all equal weight (it's much easier in a novel, which can be much longer), especially when you're dealing with three human actors who will--naturally--give different levels of performance.
One can only wonder how it might have been if the Harry Potter novels had been turned into a miniseries instead--like the Game of Thrones books have been. Turning a single novel into 10 hours of material instead of 3 hours obviously gives you more time to dedicate to each character and the unfolding of the plot can be handled in a slower, richer and more fulfilling way. It allows you to explore each character's motivations and thinking in a way that I feel the Potter characters deserved but didn't get from their film interpretations. Who knows, maybe someday someone will re-interpret it that way!