Design your characters as living characters before narrating them. Every character 'ideally' has a bio: name, appearance, distinctive physical attributes, personality, speaking style, beliefs, dislikes/traumas, age, gender, family background, and relationships.
The writer's job is to narrate the characterization in a living narrative through symbolism, action, monologue/dialogue, visualization, and atmosphere within the story.
Let your characters interact with their world organically to show who they are.