They played down Cho and Harry’s relationship a lot in the movie. There’s quite a bit more to it in the book. They actually left out quite a bit from the book, and they glossed over a little bit that’s very important knowledge later on. They also completely left out the part about Luna’s father and The Quibbler and how they worked to clear Harry against all the slander The Daily Prophet was dishing out.

(spoilers – Show)

When Harry runs after Bellatrix after she sends Sirius beyond the veil, he attempts to use the Cruciatus Curse on her and it fails. It’s clearer in the book why — she laughs and tells him that you have to mean it when you use one of the Unforgivable Curses, otherwise they don’t work. In the movie, Voldemort whispers this to Harry after the curse fails, but I feel like they made a bigger point of this in the book.

I haven’t read the final book yet, but you already know what happens in the sixth book. If you think about it, Snape could have used that curse without meaning it to allow the “murder” to be feigned. I am convinced this is what happened, but I still have to read to find out.

(I hope that bit got blocked out, but I can’t preview my comment. I guess if it doesn’t, go ahead and read for your own edification and then filter it out. =) You should really read the books, as they contain much much more than the movies can hope to express.)