I learned the hard way to figure out the details of my important characters BEFORE starting writing. This is not the same as outlining, which I see as largely optional. No, this preparation is essential.
So, before writing anything, know who your main characters are. And then for each, think through as many identifying details as possible. The obvious ones: What do they look like? How old are they? What are they interested in? What are their relationships to the other important characters? Do they work? Go to school? Do they have problems with parents, spouses, children? How about difficulties with addictions, work, health? We all have challenges, flaws, shortcomings. What are those of your characters?
Those are all valuable to know, but perhaps most importantly, how do they speak? Do they have an accent? How do they express emotions? Are they erudite or crude in their choice of words? Do they have phrases, words, they commonly use? I think nothing reveals a person as clearly as their speech patterns and those need to be clear, defined and consistent.
And you need to know these things before you begin your novel. If not, and you have written all or most of your draft, you will undoubtedly have to do a lot of editing and rewriting. And that will eat up a lot of time.
Here’s one example. In my latest, as yet unfinished book, Empty Luck, I have a prominent character with a very distinctive way of speaking, one which makes her instantly recognizable and separates her from all the other characters. She hails from East Tennessee. People born and raised in that region of the country speak with a kind of rapid drawl and very distinctive words and phrases. My problem was I did not consider this until I had finished writing tens of thousands of words. I realized I needed to research those speech patterns and then rewrite many sections of dialog. Changing the speech of such a prominent character took me quite a few hours that could have been better spent elsewhere.
It’s a simple lesson: Think ahead, in detail, about your main characters, especially the way they speak.