I think that's important to keep in mind when telling a story. It's so very easy to fall into a mindset that the only thing going on in the universe is the story you're telling. Every character springs into existence when they walk into your scene and ceases to exist the second they exit the page.
Even though there are moments when the most important thing going on in your world are happening in your story, that doesn't mean it's the only thing happening or the only thing that's ever happened in the lives of the people involved. When you get the feeling that it is, the world becomes smaller, shrinking until I stop caring about the outcome because the world you're trying to save is paper-thin.
Because I'm in the habit of naming things, I think of it as Sociopathic Storytelling Syndrome.
For the sake of keeping it all in my head, I created the acronym S.L.O.W.L.Y. to keep in mind the six things I want to know about every character that has more than a passing mention in my stories: Secrets, Loves, Origins, Wits, Learning, and Yearning.
If you know these six things about a character -- even if they never come up -- just having it in your head will bleed into their dialogue and their actions and breathe a larger life into your story. Because when that waitress leaves the scene, she's going somewhere, doing something, loving someone, or yearning for them.
And that's the kind of thing that brings a world to life.
- Secrets: Often what's unsaid forms us more than anything else. You should know at least a few things about the character that have nothing to do with the story: What are they passionate about? If you walked into their home, what would stand out about them? Are they shifty or a straight-shooter? Know the character's secrets and you'll know the character too.
- Loves: A character's gender preferences and relationship status would and should tie directly to how they interact with other characters. What or who does the character love? Are they in a relationship or are they looking?
- Origins: We are where we come from. Where was the character born? How were they brought up? What kind of family circumstances did they grow up in? Are they lonely or a loner?
- Wits: How smart is this character? This isn't the same as education and often more interesting if their smarts comes from a place of experience rather than academia, so you can tie 'Wisdom' in here too if you like.
- Learning: The demeanor of even the most incidental character will at the very least display how educated they are? Did they go to college? Drop out of high school? Home schooled? Hold advanced degrees?
- Yearning: Everyone has a goal, something they yearn for. What does the character want? What drives them? What are they willing to risk anything in order to obtain or defend?