One of my writing students just asked, “What do I need to develop about a character’s background story even though that character is not the main character?”
For characters other than the main character, a background story is more about how they came to be where and who they are today. No development of personal issues is needed. But, a story can be enhanced by giving each of the principal characters a true back story, even if it is sketchy, showing that each character has his own path that created a central potential for change within them. Then, as main character in their own sub story, each character may change (or not) in regard to their personal issue, adding interest and detail to the work as a whole. In addition, characters may be so wrapped up in their personal stories that they will choose to act against their type in the main story because of strong personal needs. For example, in the original Star Wars movie, Han Solo agrees to help rescue the princess from the detention block. This is completely against his character type (the skeptic) in the movie as a whole (out for himself, doesn’t believe in the force, etc.). Why? because Luke says, “She’s rich…” and Han needs to pay back Jabba the Hut to get the bounty hunters off his back – his personal sub-story.
To summarize – characters other than the main character need only a thin description of how they came to be who and what they are when the story begins, and this can be dropped as additional interesting exposition to humanize them over the flow of the story. But giving them a complete sub-story, even if loosely drawn, will show how their motivations were developed, providing reasons for the drives they exhibit in the story and also giving them the opportunity to grow and change.
Read more of my tips for story development at http://storymind.com