Backround Characters are Love.
And I can explain why.
Take Mike Chang, for instance. He's an asian dancing jock on Glee. He dates Tina, and has amazing abs. And that's all there is, right?
WRONG.
From the conversations Tina and Mike have (fighting over why everything has to be ASIAN) it seems like he's quite invested in his culture--after all, he met Tina at Asian Camp! When he and Tina are making out in the classroom, it is Tina who has to use Beiste to cool down; perhaps Mike wants to take it slower than she does? His parents don't want him to play a transvestite. Maybe he comes from a more conservative family? His abs are intense and he's on the football team, so we know he likes sports and probably exersizes a lot. He loves to dance--but where did that love come from?
There's more to him than meets the eye to help develop the character, but there's enough room to explore. For instance, my Mike Chang is a sweetheart with ADD (he's so quiet in Glee because he's off in his own world, unable to pay attention as much as he should). His mind wanders, and he makes connections in his head that others can't. He's creative, and smart--it's just hard to see through the fog that is his mind. He dances as an outlet, because it's the only thing he can really focus in on. He has tons of boundless energy. He adores Tina, and wants to be friends with Artie, too.
Or, maybe Seamus Finnigan. I've just started roleplaying as him on tumblr with a big group. We know some of Seamus' traits: he loves the Kenmare Kestrals quidditch team, his Mom is a witch but his Dad isn't, he's Irish, and he's close to Dean Thomas. My Seamus is all those things, but he's also loyal (a Gryff trait I think he'd have, even if he did have that momentary lapse during fifth year), kind of a ladies man--or at least, he wishes he was. He has dogs back home, and watches Muggle movies because he loves Megan Fox.
So much more than the book.
Backround characters ar a wealth of unused character development, and us, as fanficcers, as writers, as curious people, can take these characters places they've never been. And I'm excited to go with them.