All genres do it.
What? You believed those incredible bedroom gymnastics in the last romance you read? Foolish Human! Don’t you know you’d need to be a octopedal invertebrate to manage some of those positions? And the non-sexual romantic parts of the story were even worse.
Now, outside of romance and SF/F, most authors try to keep some credibility. But that’s it. _Some_ slight credibility. Yes, a few experts can shoot like that, fight like that, crack safes like that, climb like that, charm women out of the their clothes and their top secrets simultaneously . . . We writers can make our characters do it all. While they ought to be prepping for the orals to defend their Quantum Physics PhD thesis.
The tone of the book rather determines how over the top the heroes—and villains—can be. Light hearted? Camp? No problem. Dark and Gritty? Meet the minimum required level of psychological problems, and anything else goes. Military? Gotta be realistic—but you can also have a team, that among them can do anything to awesome levels
And the equipment your characters have need to be congruent with your fictional world. The hero of your medieval historical adventure can have the finest horse that ever lived. He can’t have a motor cycle.
Your characters need to have normal human feelings and thoughts. Which is nice, because you can use them to explain away some unreality. “I suppose I ought to have felt guilty about stealing money from the government. But after you’ve seen the world destroyed a dozen times, the flimsy, ill-managed financial systems of the past just don’t seem important enough to worry about.” This for my time traveler. He regularly hacks the IRS’s computers and issues himself large refunds. Otherwise he’d have to get a job. No matter. The next time he changes the past, it’ll all disappear. (I did mention that the more over the top, the less the writer needed to cling to reality, right?)
So . . . Genres. What reality, and what unreality can you get away with in the different genres? What is absolutely essential? What is unforgivable?
Westerns: Cowboys, horses, cattle, gunfights, outlaws, and marshals. Well, yeah, the setting is very important. And there is often a romance subtheme. But the main thing I see, is men standing up for themselves, doing what is right, doing what must be done. You leave out honor, independence, and stubbornness, and the book goes against the wall.
Fantasy: I have a vague recollection of fantasies that didn’t involve magic, but I can’t offhand name one. Now, the magic may just be the existence of dragons, werewolves and vampires, but _something_ other than a completely imaginary setting needs to be in there. Some of them have a ton load of reality as well. Real places, real people, real happenings. With a thread underneath of the uncanny. A reader expecting a fantasy won’t like it when you explain it all away as smoke and mirrors. Ha! Fooled you . . . what do you mean you don’t want to preorder my next book?
Science Fiction: Spaceships, Aliens, other planets, time travel. Even a few things that could also be called technothrillers. But don’t think that just because you can handwave FTL and time travel and make up Space Aliens to suit your story needs that you can get by dissing Plank’s Constant. Nope. Under all the Big Wow stuff, you have to stick to reality in the known science details or the readers will kick your ass and refuse to buy the sequel.
Mystery: You need a crime to solve. Or something to investigate. And then you have to solve it, else the readers again get peevish. You can have a fast paced thriller type (frequently considered a separate genre) or an intricate puzzle to work piece by piece. You can have romance, comedy, or tragedy. All of the above. You’re main detective can be police of some stripe, an amateur sticking his or her nose in, the accused or whatever. They can be Vampires, Space Aliens, Romans, Medieval Monks . . . But the clues really, really need to work, else, book against wall.
Romance: There has to be a romance. Really. Apart from that, anything goes. _Anything_.
Historical: Umm, needs a historical setting but the dividing line between contemporary and historical is a moving target. Right now . . . I’d call anything pre-personal computers historical, simply because the setting and details have to be _right_. Your MC, in the middle of a Vietnam War protest does not pull out her cell phone and take pictures. Even though most Baby Boomer remember the war, even if they weren’t in it, it is now history, and you have to get the details right.
A lot of the problem with getting the setting right is the balance between what a historian knows and popular perception. When there’s a vast canyon between, the writer needs to toss a rope across and walk that tightrope to try to inject reality into popular misconception.
Horror: Well, scary, Duh. Just enough off regular reality to be plausible enough to be scary instead of ridiculous. I’m not an expert on this. If you want to write it, read it and see how the best do it.
Erotica: Do I really have to say SEX!!!!! Lots of SEX!!!! Oh, a bit of plot never hurts, so long as it doesn’t get in the way. But reality has no place here. Heh . . . check out the cute ass on that Velociraptor over there . . .
Now there are these semi genres that Dean Wesley Smith calls umbrella genres. YA and Christian are probably the two largest. They are focused and framed for specific audiences, with specific requirements. Young Protagonists, Devout Christian protagonists who walk the walk. Beyond that they can be Mysteries, SF, Fantasies, Westerns, Horror . . . yes, even Erotica.
All stories, in some way step outside reality. But most of them have to stay close enough that the readers engage in the stories. They need to identify with the characters, so the better the reality of the characters . . .
Look, they don’t even have to be human. Go see Guardians of the Galaxy, if you don’t believe me. They just have to be _people_. People that the readers empathize with. That’s the one reality you really need to keep in mind.
Here, have a free short story. A quick toss off, with no research, beyond what I already knew. Where did I step in and out of reality? What irritated you because I got it wrong?