More often than not, characters are what will carry your story. It’s rare, however, for a character to be exactly the same at the end of a story as they are at the beginning. Just as carefully as you plot out the course of the story, you need to tend the growth of your characters. There’s a difference, you see, between growth, change and total derailment.
Growth is the most natural and gradual progression of a character. It’s the basis of a coming of age story. Luke Skywalker wouldn’t be anywhere near as memorable or effective as the hero of an epic trilogy of space operas if he didn’t start out as a whiny farmboy kicking sand as he dreams of life among the stars. In the new Star Trek movie, Jim Kirk militantly avoids any sort of real responsibility until he’s prompted to join Starfleet Academy, and from there he begins to grow. Scott Pilgrim may have remained an immature self-centered jerk leaning dangerously towards hipster territory if he hadn’t fallen for Ramona. Marty McFly, John McClaine, Tony Stark, Peter Parker… I could go on.
Change happens more often to supporting characters. To see the difference, take a look at what TV Tropes likes to call the Heel Face Turn. A good example is Captain Renault from Casablanca. A flattering French police captain who ‘goes with the wind’ and tries to make the Nazis comfortable so they don’t kill everybody, towards the end of the film he realizes the folly in working with Vichy and moves towards Free France. Likewise, the Operative at the end of Serenity is helped to realize what the Alliance is really all about. The difference between characters growing and changing is that growth is gradual, while change is more sudden and sometimes less than adequately explained.
Derailment is what happens when you don’t plan a character’s arc or give no basis or foundation for their change. There’s an establishing moment in Star Wars for example, when Han shoots first. If you don’t remember that, maybe you’ve only seen the “Special Edition” released in 1997 where a quick edit turns Han Solo from a cool, skilled gunslinger who gets the drop on an overconfident blabbermouth to a guy who got extremely lucky. Take a look at characters in MacGuyver, 24 and Bones to see how writers take one aspect of a reasonably well-rounded character – Mac’s inventiveness, Jack’s badassery, Booth’s boisterous tongue-in-cheek nature – and blows it way out of proportion for one reason or another, sacrificing real growth in favor of a disproportionate caricature. Maid Marian in Costner’s Robin Hood is shown to be more than capable of taking care of herself, provided it’s not the Sheriff on top of her – then all she can do is call for Robin. The first Alien depicted the xenomorph as a dark and mysterious creature with frightening intelligence and perhaps even odd fetishes. Every subsequent film has dialed down the mystique and sexual undertones to the point of showing them as generic alien baddies to be chewed up by automatic weapons fire. The number of examples of character derailment that exist show that for every good decision made by writers concerning the growth of a character, there are at least five bad ones.
When you’re getting ready to get a character started in your story, it pays off to take a bit of time to figure out where they’re going. Even if the tracks are invisible to the reader – and sometimes this is even preferable – lay them out and make sure they’re solid before you begin. Because something coming off the rails usually ends in a nasty mess.