It needed lighter bits to contrast the very dark bits. There are parts in the film - although I find it hard to find anything that I personally do remotely funny - where I've watched a scene I'm in and found myself laughing at it.
In the Homework Club scene, Rupert [Grint who plays Ron Weasley] and I are talking about how to get dates for the Yule Ball. We're not supposed to be talking in there, so Snape keeps coming up behind us and smacking us on the back of our heads with books, which is incredibly simple but very funny.

i thought that bit was gas!