1. They’re motivated.
“Every character should want something, even if it is only a glass of water.” Kurt Vonnegut said it best—the key to writing someone compelling is giving them a motivation that drives their actions and reactions. Motivations can be large-scale and grand, like the Lannisters’ quest for power, or simple and elegant, like Arthur Dent wanting some tea. Motivations add a third dimension to otherwise flat characters and will drive the majority of the plot.
2. They’re consistent.
Have you ever read a story and found yourself pulled out of the action because a character did something that just wasn’t them? Novice writers are sometimes guilty of having characters do things only to advance the plot, which can be jarring and interrupt suspension of disbelief. When in doubt, keep returning to the character’s motivation. If someone is doing something out-of-character for them (like a girl who has stage fright singing in front of an audience), show their transition and the events that forced them out of their shell. Just remember that all people have limits, and fictional people have limits when it comes to believability.
3. Their habits come from their backstory.
For every character of significance, it helps to write out a backstory for them, even if that is never revealed in the story. This way, you have something that will inform their decisions and actions, as well as humanize them. For example, a character who grew up in a poor home might be very frugal or offended when someone wastes food. A character who went through an abusive relationship might flinch when someone raises their hand. Think about what a character has been through, good and bad, and give them some habits they would have picked up along the way.
4. They’ve got little flaws.
Tragic flaws make tragic heroes, but everyday flaws make people human. Readers will empathize with characters who have trouble remembering names, or keeping their rooms clean, or get really nervous before speaking in front of a group of people. Harry Potter could be arrogant. Sophie Hatter could get frustrated quickly. Even in the grandest, most epic story, readers will be more compelled to follow characters who have little flaws just like they do.
5. They balance other characters — and other characters balance them.
How does your character interact with a nitpicky boss as opposed to their laid-back best friend? Make sure you have ample character foils to bring out different sides of your character’s personality. How differently do they treat people? How do they react when interactions don’t go their way? A balance of diverse, intriguing characters who play off each other will strengthen your story and make it something readers can’t put down.