Characters. What is it that really separates the good from the bad? What characters make the reader salivate over every appearance, cause the reader to psychoanalyze every thought and relate to every action?
I don’t really know that I could say. However, I’ve been reading Worm by John McCrae a.k.a. wildbow (Google it if you want a long and amazing read, full of detailed action, violence, and moral relativity) and I noticed something about the characters that I was reading about, thanks to the structure of the story.
If a character is developed well, it would be possible to write a story with them as the main character.
In Worm, there are interludes, single chapters or arcs that follow other characters, who either have been introduced or will be. Without exception, I find myself wondering what a full book about that character would be like. Every character has their own quirks, their way of thinking and talking, desires and ideals that drive them, strange appearances and connections, unique ways of approaching problems… and so on. It’s not just the interlude characters either. Nearly every named character in the story that shows up in person is worthy of anything from a short story to a novella to their own series.
Are my characters as good?
Could I take this man I met in chapter five and write a book about him, following his life and circumstances? Perhaps he is a normal person, hardly worthy of a book. Perhaps just a short story then. Is he interesting enough for my readers to read about him or her alone, without worrying about the impact on the plot? What can I do to make this character a character that people would write fan fiction of?
Make characters that could carry their own stories. It sounds impossible, but I’ve seen it done before. If a character could carry their own tale, you no longer have to worry about defining their role. They will carry the plot forward for you, and they will entice the readers’ mind.
So do yourself a favor and create a world of main characters. They’ll help you to write the story, and help us to read it.
Kyle Adams 
Yes! I try really hard to do this, give everyone at least one one little trait that makes them memorable. I had one character, there was several really hyped up scenes, and when it calmed down, I realized neither of them had eaten in a while. So I set the next scene in a restaurant and my female character just started gobbling down food. My beta reader thought it was funny, so it became her trait. Now I have several scenes where she’ll pull a snack out of her pocket or is trying to find a Burger King in the middle of the everything.