Without repeating too much of what has already been said on this topic, I'd like to talk a bit about how to work with canon characters. In my experience, fanfiction as a whole can be a great way for rookie writers (like me) to learn the craft in a fun, practical way. Working with established characters implies certain restrictions on what you can have those characters do and say, but it offers a blueprint that you can build upon, which tends to make the writing life easier for a beginner.
As a general consideration, canon characters are not really that different from original ones. They may be "already made", but you still have to deconstruct them first in order to see who they are, and then put them back together in your own story, which usually confronts them with new situations. Therefore, you need to know and observe their canon personalities, backgrounds and behavior, but you also have to use your own imagination and whatever real-world resources you have in order to have these characters work believably within a storyline that is of your own making.
Whether you want to explore an alternative plotline, or to expand upon something that was mentioned in canon but not detailed, the best way to start once you have an idea for a fanfiction story is to do a little research into the characters' backgrounds as laid down by their original creator. Some authors have written extra material about their mythos and characters, such as J.R.R. Tolkien, for example. For TV/movie franchises, there usually are official character profiles available. Keep all that close to you at all times and check whenever you feel unsure about one aspect of the character or another. There are also fan-made wikis out there detailing characters' histories, personalities and relationships, which can be used in the same way. You can use all that information to extrapolate what they would think or do, and what choices they would make when faced with the situation that you have in mind for your story.
For those who write fanfiction based on TV/movie franchises, you're in luck because the actors do a lot of your work for you. Watch how they move, how they speak, look out for mannerisms, whether they are speech patterns, a certain register they are inclined to use (for example, one of my main canon characters, Thorin Oakenshield, speaks in a more archaic register), or facial expressions and other physical reactions that they might display. Whenever you feel that you are losing touch with your character, go back and watch the actor play them. It's a lot easier to get back into a character's mindset if you can see them move, speak and emote in front of you. You can even look at other roles of the same actors. Even if they play different characters, actors do have a style, and their personality and style often gets infused into the characters that they play.
Actors usually do their own profiling of their characters prior to filming and even add their own ideas to them, so you can get a lot out of interviews where they discuss characters and their process in bringing them to life. This is great especially for characters that either don't get a lot of screen time, or characters that are harder to relate to. To give an example, in chapter 2 of my current story Days of Agony, I wanted to explore the relationship between Dwarf King Thorin Oakenshield and his sidekick, Dwalin. In order to get a better grip on what connects these two characters, I drew upon interviews with Richard Armitage and Graham McTavish where they talk about their characters' sharing a bond of deep loyalty and friendship.
Looking at the actors' play is also a great way to get ideas for fanfiction. We can all agree that a good character is a multidimensional one, but not all sides of them get to shine on screen. If there are hidden nuances that we only get glimpses of on film, those can be a good starting point for fan fiction. That is the case with my Hobbit series, which spawned from my desire to explore a softer side to Thorin Oakenshield that lurks just beneath the surface of the character as portrayed by Richard Armitage.
Now, when all else fails and I really can't get inside a character's mind, I do something I call exploratory writing. It's a bit like putting the character through a therapy session. I pair them with another canon character that they can open up to in conversation. It can be someone wise (like Gandalf, in the Hobbit fandom) or someone close, like a friend or a relative from canon. In that setting, the character actually starts speaking for themselves. You can do that solely to have them answer questions for you, or you can also use pieces of the resulting dialogue in your stories.
All of these are strategies that you can use in order to write canon characters, but the main things you need in order for all of this to work are not specific to fanfiction. Rather, they are actually part of the process of creative writing itself. And this is where you really learn how it's done. You need to love your characters enough to know them and respect them, and you need to actually do a bit of acting of your own. Become the character that you are writing about, and you have a real chance to write them well.
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How to Write FanfictionNon-Fiction
How to Write Fanfiction is a writing resource that contains tips and tricks on crafting fanfiction stories - by the community, for the community. This guide will serve as a helpful point of reference for fanfiction writers both old and new. We ultim...