The process of building layered, believable characters and showing how they change - or resist change - throughout a story.
Character development has two meanings in writing, and both matter. First, it's the craft of creating a character - giving them a personality, history, desires, contradictions, and a voice that feels real. Second, it's the change (or deliberate lack of change) that a character undergoes over the course of the narrative. Great character development means your characters feel like people who existed before page one and will keep existing after the last page. They surprise you, they contradict themselves, and they grow in ways that feel inevitable in hindsight.
Character development is what makes readers care. You can have the most inventive plot in the world, but if your characters feel like cardboard cutouts, nobody will finish the book. Well-developed characters carry readers through slow sections, make them cry at climaxes, and keep them thinking about your story long after they've put it down.
Connell and Marianne's development is tracked through small shifts in how they communicate, revealing years of growth in subtle behavioral changes.
Amir's development across decades - from cowardly boy to courageous man - is built through accumulating guilt, regret, and eventually, action.
Both Elizabeth and Darcy develop by confronting their own biases, with Austen tracking the precise moments their self-perception shifts.
Stevens' development is heartbreakingly subtle - the reader sees what he cannot, as small cracks reveal the emotional life he has spent decades suppressing.
Don't write 'She had become braver.' Write a scene where she does something she never would have done in chapter one, and let readers feel the difference.
Even secondary characters benefit from a few specific details and at least one moment that reveals something unexpected about them.
Backstory informs development, but development happens in the present story. Focus on choices, reactions, and change in real time.
Real people backslide, make the same mistake twice, and have good days followed by terrible ones. Let your characters be messy.
Take a character you're working on and write two short scenes set months apart. In the first, show them reacting to a minor frustration - a rude barista, a parking ticket, a cancelled plan. In the second, put them in the same situation but after they've been through the events of your story. Don't explain what changed. Just let the difference in their reaction speak for itself.