Characters are the soul of a story, and the more clearly you can paint those characters, the more believable they (and your story) will be. So how do you do that?
1. How they talk
The more character you can convey through dialogue, the better. Make sure your characters don’t all talk the same way. Teach yourself to write unique speech for each character by listening to real people around you. How do your friends talk, in contrast with your parents? How does your boss talk as opposed to the guy in the next cubicle? Do they have an accent? Do they use certain phrases a lot? Do they tend to focus more on the technical side of things, or the emotional side? Are they sarcastic? Passive-aggressive? Non-confrontational?
2. How they move
Body language is a great way to Show, Don’t Tell. She stared at him as he struggled to find words. His eyes darted around the room as he looked for an escape. She push her hair behind her ear as she blushed. He folded his arms and shook his head as he surveyed the damage.
3. How they act
Actions speak louder than words, even when you’re using words to describe those actions. I’m always tempted to write things like; “He was a generous man,” but that’s telling, not showing. Show he’s kind by describing a specific act of generosity, like paying for the groceries of the woman behind him at the checkout.
4. What they think and feel
Use your character’s thoughts to convey their deepest desires, and what they learn throughout the story. Thoughts can greatly enhance any of the above three methods:
Your character might be furious her father, and might think out a whole paragraph of passionate things she’d like to say to him. But maybe she’s intimidated by her father, so all she gets out is one weak sentence.
If their words are the opposite of their thoughts, make their body language match their thoughts. The child says he didn’t steal the cookie, but drops his eyes in shame.
And third, of course, thoughts convey motives. Maybe by reading the thoughts of our generous man, we find out that he is generous now because he feels guilty about someone he hurt in his past.
5. What they look like
I listed appearance last, because many writers rely on it too much. Avoid the laundry list description; “He had blue eyes, blonde hair, and a medium build.” Instead, think about the things you notice the first time you meet someone; you really just get a general idea of what they look like. Convey a sense of appearance, rather than a list of details; “Scraggly yellow hair and a grin that seemed almost too wide for his face.”