I read somewhere that a minor character should be a main character – of a different story. I wish I could remember who said it first, because I agree with it whole-heartedly.
In order to keep this in mind, it’s good to think of the world of your novel as a whole world. Just as you may not fully understand the lives of many of the people you interact with daily, your audience doesn’t need to know all the gory details. If a character is written with his life outside of the main story arc in mind, depth with be added automatically because you (the author) know what he’s doing in his down time.
Does this mean spending hours and hours developing unwritten story arcs for every single minor character? Of course not. But have a look at your character list, choose the minor characters who you think will be most prominent, and spend a few minutes thinking about what they do when they walk off the page. Don’t let them go home and pack themselves into a box until the next time they are blessed to interact with your story. (Unless you’re writing a story where the characters pack themselves into boxes at the end of the day and then by all means, tell us why that happens!)
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