If your story has any sort of conflict at all (which it should), your character needs motivation. They want something, for a specific reason. Maybe two characters want the same thing, but for different reasons. For example, let's say Mary and John are both searching for a lost treasure chest. Mary wants this treasure so she can pay her baby's hospital bills. John wants this treasure because he has followed in his older brother's footsteps his whole life and desperately wants to accomplish something on his own. Both characters want the same thing, but because of their different motivations, they're going to have different approaches.
If your character wants something, but there's no motivation behind it, your character will be flat. Your story will be flat. Also, when thinking about your characters' motivations, make sure you consider their needs vs. wants (full article on needs vs. wants), and how they might clash and create more conflict.
Check for motivation: Write down, in one sentence, what each of your main characters want, and why they want it. This shouldn't take more than one sentence. If you can do this right away, with little trouble, you probably have a good idea of what your character's motivations are. If not, you need to sit down and figure them out as soon as possible.
Every person has a different way of interacting with other people. So should your character. Maybe your main character hates argument. Maybe they have a temper. Maybe they're shy and don't like interaction at all.
Also, each of your characters has a different relationship with each other character. Maybe they have one certain friend they go to in order to have a good time, but another they go to for advice. They share their secrets with one character, but not another. Or maybe they don't share their secrets at all. Anything like this is a form of interaction between your characters.
Check for interaction: Make a list of all of the characters that play a fairly prominent role in your character's life. You don't have to list each and every character, but don't leave out any important ones, either. After you've made this list, consider each character on it. Write down a sentence or three describing your main character's relationship with them. How do they view this character? How do they interact?
If you can do this fairly easily and all of your descriptions don't sound the same, you probably have a good hold on how your character interacts. If not, sit down and think about it for awhile.
How can I work on this stuff? Do you have links that might help?
Yup, actually, I do have links for you. Glad you asked. Okay, maybe you didn't ask, and my bolded letters did the job for you. Either way, I have links.
Character needs vs. wants: Your characters have things they want, and things they need. These wants and needs often clash. If you aren't aware of the difference between these two things, and how you can exploit them for the good of your plot, you should check it out!
Character Words: Explained: What do I mean when I talk about relatable, likable characters? This post explains.
The Giant Form of Doom: A form you can fill out to help make sure you cover everything when it comes to character development. Highly, highly recommended. And the "of doom" is just because I liked the sound of it. No doom involved. Probably. Well, now that you mention it....
100 Things Activity: This is a simple activity that does wonders for your character development.
If you make sure to develop these four points, you will have a fully fleshed-out character. And if you have well-developed characters, your book will be better. And your readers will thank you for it.
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