Before I write a single word, I will spend a few days developing the characters in my book.
This is crucial to part of your research. Before you start, you need to have a really good sense of who these people are. Even if it means you are noting down information the reader won't ever know, you still need to be aware as the author where they come from, what their background is, and why they do what they do.
If you have well-developed characters that you know inside and out, they will write the story for you.
What's more, it helps you to add subtleties to your character that add so much to your story. Even just little things. For example, maybe in your development you find out that your character has a slight fear of spiders - which could be a brief reference in your novel, such as "Jude walked through the corridor, veering to the right so as to avoid the cobwebs." Even if it's as fleeting as this, these small bits of information capture the moments that provide the essence to your character.
So how do you develop your characters?
You do a few exercises. Here are a few that I tend to use.
Every story is based on a protagonist who wants something, and has an obstacle in their way.
The Beast from Beauty and the Beast
Wants: to be a human again.
Obstacle: he has to get someone to fall in love with him before the last petal falls off the flower.
Luke Skywalker from Star Wars
Wants: to save Princess Leia.
Obstacle: he has to brave the death star to rescue her.
Katniss from The Hunger Games
Wants: to keep her sister safe
Obstacle: she has to brave the hunger games to do it.
At first, you should make sure that you have these established.
Role on the Wall
Draw the outline of a person, like a gingerbread man, on your page.
On the outside you write as many words as you can to describe the character's external characteristics - such as the way they present themselves, and the way other people may see them.
On the inside you write as many words as you can to describe the character's internal characteristics - this is the way they see themselves, the way they feel, things that aren't obvious by looking at the person.
Above, you can see the role on the walls I did for The Art of Murder - on the left is Victor Crane's, the psychopathic antagonist, and on the right, Sean Mallon, the hard-boiled antihero protagonist.
Timeline of Life
Draw a line across the middle of the page, counting from 0 to the age that the character goes up to in your story.
Highlight the key events of their life. Include all the key turning points, and anything else you can think of that happens, however significant or insignificant.
Above, you can see mine for Victor Crane from The Art of Murder.
Complete the following list about your character's key information.
Life-views / Education
Outlook on world:
Who are they?
Where are they?
When are they?
What do they want?
Why do they want it?
How will they get it?
What do they need to overcome?
Below, you can find my key information and role on the wall for the character of Gus Harvey from Finding Her.
So, there you go a few exercises to help you get to know your characters better.
I will always do these exercises for at least the protagonist and antagonist before the first word of the first draft is written, and this is something I highly recommend - you will find yourself improving the characters you write with the knowledge you have about them.