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.
“The other night I ate at a real nice family restaurant. Every table had an argument going.”
“When I was 6 years old, I was in a rock band that was horrible called 'Dead End.' The name kind of described us. People liked us; we would go and perform at coffee houses and stuff.”