The most obvious function of minor characters is in advancing the plot.
In mainstream novels, the contribution of minor characters to the plot is very important. Virtually any element of crisis or resolution can be enhanced by a minor character.
A second major contribution minor characters can make to a novel is to enhance the development of the main characters. We learn much about main characters only from main characters - from seeing them in action or from listening to their interior voices. However, we can learn as much about them from the way they behave in encounters with secondary characters.
Minor characters can also contribute to the development of suspense, either in a scene or in a whole novel.
Another way minor characters can be useful to fiction writers is to help evoke a sense of place and atmosphere. Minor characters can be interesting in and of themselves. A quick sketch of a minor character can (and should) be vivid and entertaining - should stand on its own in some way.
Minor characters grow in surprising ways, demanding more attention from the author during revisions, competing for greater roles as novels take shape.
**Significant Roles Minor Characters Can Play in Fiction** (keep in mind during the revision process.)