In a new interview with Harry Potter and the Order of the Phoenix screenwriter Michael Goldenberg discusses the process of scaling down the movie to incorporate only what was needed for the film while keeping the spirit of the book alive.
He also mentions that this movie, the fifth in the franchise, is "more psychological and more complex and mostly more grownup."
Goldenberg on Harry in Order of the Phoenix:
He spends a lot of the film trying to work through the survivor's guilt of having watched Cedric die in front of him. One of the outcomes of that is that it really cuts him off from his usual sources of support - he's going through something which his friends can't really understand, no matter how hard they try. And the one person he really can connect to and confide in is his godfather, Sirius. So he kind of puts all his emotional eggs in that basket, so to speak. And if you read the book you where that leads.
Michael on scaling down the book:
You hate to lose anything, because it's all great. But anything that we did lose, we tried to find a way to at least tip our hats or find another way of showing it. So even where certain sequences or subplots may not literally be in the movie, I think they're there in spirit.
Thanks to UHP for the tip.