It doesn't matter what type of novel you're writing: science fiction, historical romance, paranormal, or epic fantasy. They all have one thing in common.
They have to be believable.
And to be believable, it needs to hold a degree of realism. Writing fiction grants you a lot of freedom. You can create worlds to be anything you want. Do it right and you will transport your readers straight over to another galaxy. But do it wrong and your readers will likely roll their eyes and toss your novel into their “never again” list.
Believability in fiction relies on three things: rules, facts, and complexity.
Writing fiction means you get to set the rules for your world — but there still needs to be rules.
For example, I had a conversation with another author about laws regarding child custody. Her novel is set on a fantasy world — very different from Earth and from the United States. Given my degree in psychotherapy, we discussed the different laws and procedures here in the United States about child custody: what happens during the court hearings, what sorts of things weigh into the decision for placing the child, which opinions do the judges listen to.
Now, in her world, she wanted the child's feelings and preferences to hold the most weight, whereas here the child's thoughts on the matter mean very little. So what does that mean? Does that mean her story won't make any sense?
Of course not. The rules in her world will work as long as she doesn't break them.
And this doesn't apply to Earthly happenings like child custody or gravity. Even completely fantastical elements in your stories – magic, time-travel, superpowers – all require rules. In Doctor Who, the concept of time is described as “Wibbly Wobbly” – meaning it didn't quite always go in a straight line. The three suns in Pitch Black kept the planet in constant light, and lined up to form a month-long eclipse once every 22 years.
In your fantasy world, it doesn't matter how magic works, or why people can use magic, or what you choose to be done by the magic. The only thing that matters is that once you set the rule, you stick with it. Nothing turns a reader off more than finding out that the rules apply to everyone except your hero or that for some unexplained reason your hero is fire-resistant and can fly.
Even fiction requires some facts. The most obvious Johndra, historical fiction, may seem obvious. For historical fiction to work, any facts you do choose to include must be verifiable. Doing so helps your readers to accept the unrealistic, fantastical elements you've added to the story. The same holds true for every genre.
If your characters are on the run in dystopian Chicago, and one of them becomes pregnant, you must follow the medical facts of pregnancy. For example, humans cannot sense they are pregnant within minutes of procreating. Nothing will pull your readers out faster than reading something factually impossible even if they are reading fiction. Your excuse that it is a fantasy world will not hold their interest and will not make them forgive the mistakes.
If you want your readers to believe in your story, they have to believe in your characters. They have to know what's at stake and root for your characters in a fight. Relationships, the manners in which characters are interconnected, have to feel real. If a character does something the reader would never do, then the character must convince the readers it is still a reasonable choice.
No hero is flawless, and no villain is evil incarnate. People are multi-dimensional: filled with goals and inner conflict. And at their core, heroes and villains are one in the same. They're working toward the same end: to save the world, to save themselves, to save the person they love. Understanding human nature and the possible ways in which a character will react to circumstances will help build believability in the story.
Key Point to Remember
Realism in writing isn't reserved for nonfiction. In fact, in many ways, fiction relies more on realism than nonfiction does. Readers want to believe in the worlds you've built for them. Realism will help make that possible.