Characters are the soul of a novel. Even if the concept of your plot is surviving doomsday what makes the plot a plot is the interplay between characters. I struggle to believe there is such a thing as a story that isn’t character driven. Even if the antagonist is a monster he should be treated as a character. He wants something. Make him go after it
All very well but what goes into a good character?
If you understand the aspects of your characters culture, and how his history affects the way he views the world, there is little about him you can’t figure out. And once you practice enough and experience the world enough such understanding should become automatic. That’s the theory anyway.
The world we carry with us is just as rich and complex as the external one. Race or ethnicity can change a person in the sense that encountering prejudice has a lasting effect. In that vein mental stability and experiences with psychiatric episodes, as well as deepening the understanding of humanity, also comes with stigma.
It's essential you understand where your main character was born, how he grew up, what major events shaped his life. This will affect everything he does, every choice he makes and his opportunities to grow. You don't need a full detailed list, and I've found I keep discovering my characters past as I write. But you really need to know him as you would your closest lover.
“Culture or Civilization, taken in its wide ethnographic sense, is that complicated whole which includes knowledge, belief, art, morals, law, custom and any other capabilities and habits acquired by man as a member of society.”
- James Clifford and George E. Markus,
Then there is Nature vs Nurture. Certain states of mind can change either of those. So while your character may be an outgoing charming socialite, if he's getting a divorce things may be different it is as true for people as it is for characters. But also as characters learn to cope with the stressors in their life their reactions change.
Long grueling stretches of time with nothing but conflict the entire way will take it's toll. Characters should become ragged.
Discovery
If you only have a few ideas about who is in your book, don't get bogged down, start writing anyway, just don't get glued to aspects either. Some material will never make it into the book. Just keep your fingers going and your mind open.
You will soon have people living inside your head. Just don't be too surprised when they talk back. Characters can be very opinionated about what they want to do. It's better for the story to listen to them. I know it sounds strange, but that is writing for you.
If writing is a kind of madness, the only cure is to create.