Remember when you were little and everything was done for you? Someone cooked and cleaned and fed your dog or cat. Someone washed your clothes, washed your hair. Someone brought in money and paid the bills. Someone took care of you when you were sick. Someone drove you to dance class and ball practice.
Now you're an adult. That "someone" is you.
If you're a writer, you have to take as much care of the characters you create as you do yourself and your human family.
You have the responsibility to give them life each time they appear on the page. Not only your protagonist and main characters, but the secondary and tertiary characters,too. Especially the antagonist. And even the walk-ons. Those walk-ons are there for a reason. If they're not, then they need to go. Or several need to be combined into one.
Giving life to your characters is more than writing about facts that would appear on a driver's license. Live in his or her head for a little while. Really put yourself there.
Go outside and take a walk around the block in your characters' shoes.
In the manuscript I just completed, the main character is a woman trying to outlive the man who wants to kill her. She's extremely aware of her surroundings and the body language of the people she meets. Her point of view is filtered through those priorities.
The antagonist thinks he's king of the world. He's the baddest, smartest dude in the sandbox. No one would dare try anything with him. When you take that same walk with him, you see the block from a different perspective. You don't spot the places someone could hide. You don't think about the dangers of deep doorways. Instead, you think about how stupid some people are. How they invite anyone to break into their houses and cars and don't even know it. How easy it would be to befriend one of the women out walking. Especially one woman.
Go take a walk with your characters. It'll do all of you good.