Have your characters write their own life stories
For your characters to be compelling, your readers will need to think of them as real people, not just dramatic functionaries or collections of traits.
To help make this happen, have each of your characters write a short one-page autobiographical piece about themselves in their own words, describing their childhoods, backgrounds, activities, interests, attitudes, relationships, pet peeves and outlooks on life.
Try to write these in the unique voice of each character and from their point of view. Don’t write about them; let them write about themselves.
This will give you the experience of what it is like to see the world through each character’s eyes, which will help you empathize with their motivations and thereby make it easier for you to write your novel in such a way that your readers can step into your characters’ shoes.