The reason why H.P. followed the book so closely is that J.K. Rowley had too much control over the screenplay. P. Jackson didn't have the postAuthorID looking over his shoulder, or if old J.R.R. Tolkien did, he kept his comments to himself and let the director get on with his business.
Disclaimer: I have not yet seen the H.P. movie and am still wishing she would churn out the fifth book already. Had she spent her time on that, the movie might have been better received.
