Eric Knight’s posted an excellent article on working backstory into one’s plot. Lord knows, I see a lot of ham-handed back story creation and have written some myself; it’s a subject that all world builders need to consider carefully. Just because you know all the names of everyone in Ravenclaw House doesn’t mean that you need to tell us those names. J. K. Rowling, incidentally, DOES know the names of everyone in Harry Potter’s year. But the action doesn’t stop so that we KNOW that she knows all the names. The information just gets used.