We all have habits–and those habits, good or bad, define us in the eyes of others. The girl who is always late for work, the boy who never says “thank you”, the couple who drop in on their friends–and stay for hours–without ever extending an invitation to their house in return.
We might decide, rightly or wrongly, that the late girl is lazy or irresponsible, that the boy is greedy or thoughtless, that the couple are cheap and selfish. If any one of these individuals acts contrary to the established pattern, we’re going to assume that some new element has been added to our story to prompt this abrupt change in habits. The girl starts turning up on time for work, because she wants to impress the new boss–and possible romantic interest. The boy cultivates some manners in the hope of securing a new bike from a visiting aunt. The couple begin to throw parties at the house they have just purchased.
The abrupt changes in habit can’t be defined as changes in character, because the girl, boy, and couple are still acting in ways that serve their self-interest. Once the girl has secured a date with her new boss, or discovered he isn’t interested, she will probably return to her old habit of turning up half an hour late each day. The boy will go back to putting his feet up on table and leaving muddy prints on the carpet once Aunt Martha has returned home, even if she left him that bike in exchange for future good behavior. The couple may only be hosting parties to show off their new house; once they have done so, they will go back to freeloading.
Once a pattern is established in your story, stick to it–or explain why your characters have deviated from it. Your characters should NOT change merely to fit the story’s plotting.by