There's a particular kind of reader who will finish a novel, set it down with a satisfied sigh, and immediately start wondering what happened to the postman.
Not the protagonist. Not the villain. The postman — the one who appeared in three scenes, delivered a pivotal letter, made a dry joke about the weather, and was never seen again. That reader will lie awake constructing an entire biography for him. A difficult childhood, perhaps. An allotment. A secret fondness for jazz.