I mostly write fan fiction, and therefore have read quite a bit of it.
In the Harry Potter universe, Draco Malfoy is well-known as Harry’s nemesis through most of his school years. Only much later in the books does he really reveal any redeeming qualities at all. The problem, though, is that he was represented in film by Tom Felton. Many girls developed a crush on the actor, and so when they turned to writing fan fiction in the HP world, they wanted to include Draco and they wanted him to be “good”. Now, I can deal with someone writing a storyline that drifts from what the author originally wrote, but if they are going to change the nature of the characters to any appreciable extent, they need to provide a valid reason for the change. We have seen Draco be nothing but arrogant, snotty, rude and even downright mean, particularly toward Harry and his friends. If you want me to believe that they are all now “best buds”, then you need to show me a compelling reason for that change. Why did Draco turn away from all that bad behavior and become someone who could be friends with the Trio? Or did the Trio have a compelling reason to give up being good and go join Draco in being bad?
Why does this matter? It’s part of staying true to the character. It isn’t a bad thing for a character to change over the course of time and events, but show that change happening, show what is causing it to happen. Few people make a drastic personality change on a whim for no discernible reason. J.K. Rowling understood that, and changes in Draco were incremental through the course of the book, so that toward the end, you understood a little of how and why he regained respectability. These fan fiction authors don’t ‘get it’, and they make a complete turnaround in behavior happen in the blink of an eye for no reason at all.
What about your characters? Do they stay true to how you wrote them? Do they make logical changes over time, based on things that happen to or around them? Spoiled and petulant teenage girls don’t become the Duchess of Cambridge without a lot of work happening in between the two. Muscle-bound bullies don’t become champions of the weak without a compelling reason. Gangbangers don’t suddenly beome priests without some defining event altering their perception.
When you write each character, you must first begin with knowing who they are. Then as you go along, ask yourself how they would react in each new situation they face. Yes, sometimes they can do something surprising – perhaps even to themselves – but there still should be a reason. Maybe they rose to the occasion and finally stopped whining, complaining and avoiding doing what was right. Maybe something happened earlier that affected them more than they realized or acknowledged, and not until this moment does it strike a chord in them. An example would be the Black Panther in the Avengers: Civil War movie. [spoilers]
His father is killed, sending him off on a vendetta to kill the person responsible. For a long time, he is pursuing the wrong person (who has been framed for the crime). Then when he finally comes face to face with the true culprit and sees how this man was so broken by his own loss of family that HE went on a revenge vendetta, he realizes that killing him won’t bring back Black Panther’s father or make him feel any better. He sees himself in his enemy and changes his actions. But we have seen him come to this point, and we aren’t particularly surprised by his choice.
So look at your characters closely. Know them. Know how they think and act and react. Keep them true to that, no matter what. And, if you want them to change, introduce catalysts for that change – some spark. Would Black Widow be seen crying? Would Steve Rogers disembowel someone just because he could and didn’t happen to like the person? I’m sure you can think of many other characters in books or on tv and movies, and you know how you expect them to act based on the character you have been shown. You know what you would find very odd about their behavior or language. We see it all the time, whether voiced by others or ourselves. A movie is made of a book we’ve read and loved. Immediately a howl goes up: That’s not who that person is! That’s not how they would act! You got them completely wrong!
Many Lord of the Rings book fan were outraged by Faramir’s treatment in the Peter Jackson’s movies. In the books, he was completely honorable and never once came close to succumbing to the Ring’s power. He gave the Hobbits aid and sent them on their way. Instead, in the movie, we see him dragging them with him to Minas Tirith, with dreams of glory put in his thoughts by the Ring. Only much later does he come to his senses (and not for an entirely clear reason) and let them go. Suddenly it looks like mental illness runs rampant in the entire line of Stewards: father, older son and younger son, with each succumbing all or in part to Sauron’s evil. Lost are the important differences in motivation and reason for their actions, and completely lost is Faramir’s utter integrity and strength of will that he didn’t succumb.
Peter Jackson’s problem was in changing the nature of someone’s character, a person known and beloved from the books. Had his movie been an original story, that person acting in that way would not have elicited such a strong response. But Lord of the Rings (book) fans knew who that character was, and it wasn’t the man they saw portrayed on the screen. That disconnect affected their enjoyment of the movie. That detail jarred the viewer out of the story. And the last thing you want to do is lose your reader/viewer because you derailed them.
Know your characters. Keep them true.