In writing stories we strive for it to have life-like qualities. It needs to feel real even though it's not. Stories cut out boring parts and focus our attention on what the writer feels is important. It doesn't communicate the full extent of everything at all times. Lots of things are there as set dressing just to make the world seem real. Some of the set dressing isn't just objects but smaller characters.

Strangers don't want you to get too involved in their life and you have no business doing so. It's similar with stories: readers have no business getting to know smaller characters really well. The only people that should be prominent are main characters because that's how life is.

Casual Acquaintances Show Normal Side First

Most people you run into in your life do their best to blend in and not stick out. The more you interact with a person, the more you get to know them and the more they reveal their true self. The more you get to know someone the better friends you are and the more prominent they are in your life. So it is in a story. The more you get to know a character the more is revealed about them thus they are more prominent. The small characters in stories, just like the guy you asked what time it was this morning need remain more like types than real people to feel life-like.

Otherwise Indicates Character Will Be More Prominent

In stories you can start with knowing unique things about a character and the rest of the story fills in all the other stuff. In life this is rarely the case. So when you meet a character in a story that is multi-dimensional and compelling we assume this character will be a big part of the story. It's dishonest and unreal to know a character well but not spend much time with them. It doesn't feel right to know too much about somebody that is inconsequential to us in the story.

Breaks Illusion If Every Character Is Too Real

Even though you might feel it's necessary to give each and every character a touch of realism, it may end up detracting from the overall quality of the story. When we go to the store with a friend, it's our story; every person we run into doesn't become a part of it or else nothing would get done. In life you have your important characters (friends, family) and bit characters (strangers, acquaintances). Just like in life there is emphasis on people that have an impact on your life and less on those who have less impact.

Don't Use It Just Because It Happened In Real Life

You may have once gone to the store and had a really personable cashier ring you up. She may have been so unique that you couldn't help but remember her years later. So when you write that scene and "write what you know" you may create a character that is just like this person, and she steals the scene and even the story. Just because it happened in real life is no reason why it need be included in your story.

Stories are like an average person's experience, but ordered and meaningful. A balance needs to be struck between reality and fictional storytelling. To help make stories seem more like real life remember the less a character is in a story, the less well-drawn they should be. Life is mostly like that and so should your stories.