Now, not every character needs character development. After all there is a reason for static characters (I like think it’s to keep authors from going bonkers ) , but if you want your character to develop and change as a person, then it’s important to make sure that they grow, or shrink, but not mutate into something else (except, maybe, literally), so make sure that their development follows logically. The terms I’m using, positive and negative character development, I made up on the spot. I have no idea what terms are actually used.
Positive character development
When people think of character development, they think of this kind. This is where a character improves him or herself in some way. It’s also the most commonly used, because most people don’t wants to see characters becoming messed up wrecks. Starting off as messed up wrecks is another story (See House).
The best way to create positive character development is to give the character a compelling reason to change for the better, like Scrooge being greeted by the three ghosts of Christmas in A Christmas Tale. He saw what he would become if he continued down the path he was going, and that made him change course and become a better person. That’s positive character development in a nutshell.
To put it laconically: a character has to see, and be made to care about, the destructive consequences of his or her actions or the benefits of a different course of action to be made to change for the better.
Negative character development
Character development doesn’t necessarily need to turn the character into a better person. Sometimes it’ll have the opposite effect Usually this involves some sort of trauma, but sometimes it’s just being beaten down again and again with no hope of change, or finding that being the nice guy simply makes things worse. For example, someone who’s consistently wronged may become a misanthropic asshole as time goes on, instead of, say, trying to find a way to ensure that others aren’t wronged. Another example would be a character that is forced to kill to survive may become so used to killing that they feel nothing, not even guilt, as they end someone’s life.
This type of character development is mainly used to create tension or drama, and when used well, it works. When used badly, it feels like an ass-pull the writer pulled out in a cheap attempt to make things more dramatic, so make sure that it makes sense and isn’t clunkily implemented
Another laconic description: If, for some reason, a character is subject to strong negative experience, or constantly subject to hard knocks, they may become more emotionally withdrawn, more depressed, or more angry.
Using character development
Ultimately, character development should be used to deepen the character and drive the plot forward, as long as it’s done logically, not make them cooler or some such, and you should NEVER make a major change in the way a character acts out of the blue just to drive your plot forward. You’ll just piss off your readers.