People continue to use the “fiction” aspect of fanfic as an excuse to write whatever they want. That’s not how it works.
If I wanted to read an original story then I would read an original story. So if you’re going to use existing characters then write the existing characters. At least to the best of your ability. That’s all I ask.
There also seems to be this misconception that placing the characters in an alternate universe gives you license to write whatever you want. Not so. While you do have more creative freedom, there are still restrictions if you’re going to call it a fanfic. Don’t get me wrong, I have nothing against AU fanfic. I quite enjoy it from time to time. But it has to be done right. Which is rare. Here’s the key: you can do whatever you want, as long as you take the time to world-build (hint: there’s a fine line between too much detail and too little) and retain the core elements of the character’s personality.
As a fanfic writer you have one main goal and that is to make me, the reader, believe. Make me believe that—given the circumstances—this could happen. That is your challenge. Appreciate it. Embrace it. Because it won’t be easy. In fact, if it’s too easy you’re probably doing it wrong.
Because these characters don’t belong to you. You’re taking something that already exists and expanding upon it.
Do your research. Get to know the characters. Watch/read everything you can get your hands on. Look for their quirks and habits and body language and preferences. Remember the little details and seamlessly weave them into your story. Little hints of reality that will add depth and continuity and make your story that much more believable. But most important, pay attention to their interactions with other people. How do they treat their friends? Enemies? Strangers? It’s not necessarily what someone says that defines them, but what they do.
First observe, then write.
Too often I encounter fanfic where the existing character’s personality has been ripped out and replaced it with a manufactured one to fit in the writer’s little fantasy world. What’s left is a hollow shell, which may look like the character and share the same name, but that is where the resemblance ends. I don’t want to read other people’s personalities with the facade of my favorite characters. I want to read them with all of their unique quirks and flaws. What’s the point otherwise?
Yes, I understand it’s hard to pair certain characters together without drastically changing things, but it can be done if you’re willing to develop them properly and not turn the story into another generic romance novel. Not everything is puppies and rainbows and cotton candy (nor is it a telenovela). I would rather read something real and heart-wrenching and introspective than a pile of fake drivel.
So how do you avoid “brain transplant” syndrome? Well the concept is quite simple actually. Do not write a story first and then place the characters in it. Build the story around the characters. They are your foundation. While you should have at least a vague idea of how you want your story to progress when you begin, always modify the story to fit the characters, not the other way around.
As you’re writing continue to ask yourself, “If placed in this exact same situation would so-and-so do this? Would they really say that?” If the answer is ever no, rewrite until the answer is yes. If you have no idea, it’s probably time for more research.
Now some would say that it’s acceptable to write something out of character as long as you give a warning and/or explain why. I say NO. The moment you deliberately step out of character is the moment your story becomes original fiction and ceases to be fanfic. If you know the character you’re writing would never do or say something and you choose to ignore this because it fits your story better, you have failed as a fanfic writer.
Well, okay, there may be one teeny tiny exception to this rule: “what if” scenarios. You might be able to get away with this if the whole plot of your story revolves around changing one major aspect of a character while everything else basically remains faithful to canon. I hate to even add this qualifier because 99.9% of the time breaking character is the worst thing you can do, but I have seen this done right on rare occasions. However, I wouldn’t advise trying it unless you’re very experienced.
Anyway, I’m sure I’m forgetting to mention something important and I also know not everyone will agree with me on this subject. In which case, I suppose you have the right to your own opinion just as much as I do. But I’m going to shut up now because I think I’ve worn my point into the ground and probably lost some followers in the process.