Look at the Coen brothers. All their minor characters are as interesting as their protagonists. If the smaller characters are well-written, the whole world of the film becomes enriched. It's not the size of the thing, but the detail.
“I'm not sure that you can say definitively that some roles are better filled by consultants, but I would say that some projects are better handled by consultants.”
“A man's illness is his private territory and, no matter how much he loves you and how close you are, you stay an outsider. You are healthy.”