When writing early drafts of books, I frequently end up with whole scenes that never make the final cut, or which simply don’t belong in the story. But these orphaned scenes are not wasted; far from it.
When talking about writing I tend to use metaphors a lot. Even though I can’t paint at all, I sometimes think of characters as different coloured paints. Not in a literal way (I’m not that freaky) but in the sense of them all being there, lined up on your palette. To write in a particular character’s voice, to paint them into a scene, you have to get into their head. But you can’t do this unless you already know them well.
Just as an artist has to mix their colours in order to paint a picture, so a writer needs to mix their characters. Much of this I do along the way, with every scene in which each character appears. After you’ve written with a character a few times, you have a fairly good idea of how they will react in certain situations, how they sound, what they say, and what they’d never say. After a while, you don’t have to think about it… you just dip your brush into the appropriate character’s colour and they appear on the page, with even the smallest brushstroke containing something of them. You even know when they’ll fade into the background.
I tend to ‘mix my colours’ as I go, learning about the characters during the messy first draft. But I’m sure it can be done deliberately too. It must be a good idea to pick random dramatic scenes from life, and write your character taking part in them. By the end of the process, you ought to have a good quantity of their ‘colour’ on your writer’s palette, there to use freely when you start writing the book for real.
The story I’m writing at the moment is an extreme example of this. I had planned an entire book, getting so far as to do a chapter-by-chapter outline, but then the story went splat (as they do). But when I cleared up the mess, one supporting character was left behind. She became the main character of my new book. And, although virtually every detail of her life is different now (even her accent), her essential character and voice remain the same. The colour is ready-mixed. If only that happened more often.