In other words, they need to change somehow; it might be that they learn from their mistakes, or maybe climb out of a situation they are thrown into and become a better person for it.
James Bond is a good example of a character, who does NOT develop from start to finish from one film to the next. He remains reliably arrogant, charismatic, charming (?) and the ultimate hero throughout. You know that whatever scrapes he gets into, he will be beaming at his latest conquest as the credits roll. James Bond films rely on extreme action to thrill us. Having said all that, I think Daniel Craig has been an improvement on the rest in this regard.
In the novel I’m currently penning, Forever Lucky, middle-class Katie is left at the end of Chapter One with no husband, two spoilt, demanding daughters who are used to a certain lifestyle, and very little money. Her character develops to meet the challenges she faces, and the story evolves through how she does this and what she becomes as a result.
So when you start your novel, think about how their character will develop to shape the story. What or who will influence them? What are they like at the beginning of the novel and what are they like at the end. It’s a good place to start.