They are going to have to start editing the books to make good movies. Order of the Phoenix has a ton of stuff which could be cut. While the stuff at Sirus's house was neat, it could all be cut. It just doesn't matter to the overall story. All that matters is that we know who are in the Order.
Part of the problem though is that Rowling is using some stuff as setup for later payoffs and until the book series is complete it can be difficult to decide what's filler and what's essential. I know Rowling consults on the films and gives advice on the scripts from I don't think she flat out tells the screenwriter whats going to happen.
Some of the stuff with Neville in the fifth book, for example, would lose impact by not having the supporting stuff about his character in previous books. Yet just looking at his presence in earlier novels you would think he would be a relatively unimportant supporting character who could naturally be cut.
And don't forget they cut out the major revelation about Proudfoot, et al in the Azkaban film. If something that big could be removed out of the relatively short Azkaban novel I do have some concerns over what might be cut in the 4th and 5th films.
But of course these must stand as movies first. The first two films suffered from perhaps too much slavish devotion to the source material. But in having to adapt three straigth 700+ page novels into three 2.5 hour films I think the filmmakers might find themselves in a bind in one or two more movies.