This essay, on “The 38 most common fiction writing mistakes”, offers advice to writers. But the rest of us can also learn useful details on how fiction can bias our thinking. Here are my summary of key ways it says fiction differs from reality (detailed quotes below):

Features of fictional folk are more extreme than in reality; real folks are boring by comparison. Fictional folks are more expressive, and give off clearer signs about their feelings and intentions. Their motives are simpler and clearer, and their actions are better explained by their motives and local visible context. Who they are now is better predicted by their history. Compared to real people, they are more likely to fight for what they want, especially when they encounter resistance. Their conversations are mostly pairwise, more logical, and to the point. In fiction, events are determined more by motives and plans, relative to random chance and larger social forces. Overt conflict between people is more common than in real life.

And I’ll add that stories tend to affirm standard moral norms. Good guys, who do good acts, have more other virtuous features than in reality, and and good acts are rewarded more often than in reality.

A lot of our biases come, I think, from expecting real life to be like fiction. For example, when we have negative opinions on important subjects, we tend too much to expect that we should explicitly and directly express those negative opinions in a dramatic conversation scene. We should speak our mind, make it clear, talk it through, etc. This usually a bad idea. We also tend to feel bad about ourselves when we notice that we avoid confrontation, and back off when from things we want when we encounter resistance. But such retreat is usually for the best.

Those promised quotes:

In more than twenty years of teaching courses in professional writing at the University of Oklahoma, I think I’ve encountered almost every difficulty an aspiring writer might face. …

“Wally, these characters are dull. What they are is flat and insipid. They are pasteboard. They have no life, no color, no vivacity. They need a lot of work. ”
Wally looked shocked. “How can these characters be dull? They’re real people-every one of them! I took them right out of real life!”
“Oh”, I said. “So that’s the problem. ”
“What?” he said.
“You can never use real people in your story. ”
“Why?”
“For one reason, real people might sue you. But far more to the point in fiction copy, real people – taken straight over and put on the page of a story – are dull. ” …

Good fiction characters, in other words, are never, ever real people. Your idea for a character may begin with a real person, but to make him vivid enough for your readers to believe in him, you have to exaggerate tremendously; you have to provide shortcut identifying characteristics that stick out all over him, you have to make him practically a monster-for readers to see even his dimmest outlines.

For example, if your real person is loyal, you will make your character tremendously, almost unbelievably loyal; if he tends to be a bit impatient in real life, your character will fidget, gnash his teeth, drum his fingers, interrupt others, twitch, and practically blow sky high with his outlandishly exaggerated impatience….

Good fiction characters also tend to be more understandable than real-life people. They do the things they do for motives that make more sense than real-life motives often do. While they’re more mercurial and colorful, they’re also more goal-motivated. Readers must be able to understand why your character does what he does; they may not agree with his motives, but you have carefully set things up so at least they can see that he’s acting as he is for some good reason. …

In real life, a young woman may come out of a poverty-stricken rural background and still somehow become the president of a great university. Except in a long novel, where you might have sufficient space to make it believable, you would have a hard time selling this meshing of background and present reality in fiction. … In short fiction, characters and their backgrounds are almost always much more consistent than people in real life.

Motivation? Again, fictional characters are better than life. In real life, people often seem to do things for no reason we can understand. They act on impulses that grow out of things in their personalities that even they sometimes don’t understand. But in fiction there is considerably less random chance. … in real life people often don’t make sense. But in fiction, they do. …

interesting characters are almost always characters who are active-risk-takers – highly motivated toward a goal. Many a story has been wrecked at the outset because the writer chose to write about the wrong kind of person -a character of the type we sometimes call a wimp. … He’s the one who wouldn’t fight under any circumstances.
Ask him what he wants, and he just sighs. Poke him, and he flinches-and retreats. Confront him with a big problem, and he fumes and fusses and can’t make a decision. …

In reality-in the real world -much of what happens is accidental. … In most effective fiction, accidents don’t determine the outcome. And your story people don’t sit around passively. … In good fiction, the story people determine the outcome. Not fate.
…

In fiction, the best times for the writer- and reader- are when the story’s main character is in the worst trouble. … There are many kinds of fiction trouble, but the most effective kind is conflict. You know what conflict is. It’s active give-and-take, a struggle between story people with opposing goals. … The calmer and more peaceful your real life, the better, in all likelihood. Your story person’s life is just the opposite. You the author must never duck trouble … Because fiction is make-believe, it has to be more logical than real life if it is to be believed. In real life, things may occur for no apparent reason. But in fiction you the writer simply cannot ever afford to lose sight of logic and let things happen for no apparent reason. …

In real life, coincidence happens all the time. But in fiction – especially when the coincidence helps the character be at the right place at the right time, or overhear the crucial telephone conversation, or something similar -coincidence is deadly. Your readers will refuse to believe it. …

Your character must have an immediate, physical cause for what he does. This immediate stimulus cannot be merely a thought inside his head; for readers to believe many transactions, they have to be shown a stimulus to action that is outside of the character-some kind of specific prod that is onstage right now. Turning this around, it’s equally true that if you start by showing a stimulus, then you can’t simply ignore it; you must show a response. … In real life, you might get a random thought for no apparent reason, and as a consequence do or say something. But … fiction has to be better than life, clearer and more logical. …

Writers sometimes mess up their dialogue. Sometimes, without realizing it, they let their characters talk on and on, boringly, becoming windbags. … The great majority of your characters have to be more terse and logical than we often are in real life, if the dialogue on the page is to appear realistic. … whenever possible, set up your dialogue scenes so that they play out “one-on-one”, getting rid of other characters (who might interrupt and make the conversation more complicated). … Simplicity… directness… goal orientation… brevity. These are the hallmarks of modern story dialogue. …

If you have any doubt that the reader will understand the meaning of what someone in the story says or does, you must work in at once some method of pointing out what you may think is obvious. (more; HT Eliezer Yudkowsky)