Week 1: The Set-up
Week 2: Conflict
Week 3: Character
Change is the essence of plot. Your characters go from point A to point B and are changed by the journey. Sometimes, even your setting is changed--Lord of the Rings is an example of a story where the entire world is changing.
In Jane Austen's world, nowhere is this concept of change more evident than in Darcy. His whole appeal really centers on his willingness to change, to become a man who knows how to please a woman.
Change is also the absolute hardest thing to write, in my opinion. Usually, when you start the story you know where your character will be at the beginning and the ending. The path of getting them there can be difficult to chart. For instance, I struggled with Darcy's post-Hunsford disposition. How long did he hang on to his resentment? When did he first begin to see things in a different light, and what sparked that change?
Change should be gradual and natural, something that flows out of your story. It is the effect of your plot, not the cause. Remember, plot = conflict that forces your character to change. Conflict is the cause, change is the effect.
In my Colonel Fitzwilliam novel, the Colonel starts out very sure of himself. However, much of his confidence in his ability to read people--very important in a career as a spy--disappears when he realizes he was betrayed. That is the first change. The second happens later, when the heroine confronts him over it. That sets up the secondary conflict surrounding the romance, which forces the third change and ends with the complete transformation of Fitzwilliam.
Do you know where you want to take your characters? Do you have an idea of the path they'll follow that will seem realistic to the readers? They don't have to be the same characters we know and love at the end of the book as they are at the beginning, but if you're going to change them, you need to show us every decision along the way that leads to their transformation.
Be the change... Um, perhaps that doesn't work so well for fiction. ;)