From an Editor: Common Writing Mistakes

Character development

Writers sometimes struggle with developing their characters—characters are too flat, or their actions don't make sense.
For example, Jason is a character with the flaw of being too cowardly. This shows in all his interactions, but once in a while he does something brave. A character like him might have a story arc where he slowly builds up his courage, and something pushes him to go outside his comfort zone. However, without that build-up and with no reason to change his behavior and tendencies, having him suddenly do something incredibly brave for the sake of the plot will seem nonsensical. It may be that you, the writer, understand what pushed him to act outside his usual boundaries, but you must remember to show what pushed him and/or his development into doing so.
Alternatively, suppose Jason never stops. Suppose something that should've worked out for him goes horribly wrong in a moment of cowardice. That's okay. But if he's the main character, something needs to change for him. If this trait remains one of his stable flaws, what else changes? Your main character needs to play an active role in developing the plot; this plot also needs to stimulate his changes and shape his story arc.
But what if you're writing a story where no one changes? Then something else needs to. Basically, a story needs to have a reason for happening, something that pushes what was normal into a tale worth telling.
If you're not sure how to answer these questions, perhaps you can step back and complete a character profile for your main characters and important side characters. Characters are the driving force of every story (even if your setting itself is a character), so they must be the most well-developed. They're what your readers will become attached to.
Remember: your characters do not have to be likable, but it's best if they're at least somewhat relatable. We become invested in those we can see parts of ourselves in. Show us their strengths and their flaws, and plan how you want your characters to use these to their advantage or detriment. We want to live with them, run with them, cry with them, and smile with them.