There are two things you hear anytime writers start going off about how to write. In their defense, of course, this is usually prompted by someone asking them - I haven't met a (decent) writer yet who will just start firing off his theories on what makes a great story.
The first is "write what you know". This is the most common thing you'll hear and probably the least understood. The second is that one's writing should be "realistic" - this one comes in many variations but the nut of all of them is that what comes off the page has to be "realistic".
Since this subject came up in the last post I figured I'd just go off about it here; I figure if people are reading this then that means I don't have to wait for people to ask me what I think.
Both of the above thoughts are connected, pointing at something that is hard to describe. I call it the "ring of truth". Fiction writing is ultimately about truth. This may seem paradoxical but it's not really when you think about it. The whole point of fiction writing, other than to entertain, is to try to get at the inner truths of human nature in a way that makes them easier to understand. Real life is clumsy, messy, and not set up in three acts. Its resolution is death with a lot of loose ends left hanging. This is why Man invented storytelling. There is something in us - I think it's a clue to Creation - that craves the clean plot, well-defined characters, and resolution of a good story. Maybe it's just because we typically want what we can't have.
I say "write what you know" is misunderstood because it's not really advice to only write about things you yourself know about firsthand. That'd be dumb advice and no one should try to do that. Probably why there are so many stories moldering in slush piles that involve a protagonist who's a struggling writer. It's also why stories that have a writer as the hero are sort of a pet peeve of mine - to me your story has to be damn good to justify that (fortunately I gave Brian Keene's Dark Hollow another shot).
What "write what you know" really means is not "knowledge" in the sense most people think of the word, which is recall of facts, ideas, things are categorizable, sort of objects of the mind. It is about "knowing" in another sense, knowledge that is hard to quantify or even describe. It's not knowing what the capital of Afghanistan is, it's knowing what it's like to stand on the plains and look off at the distant mountains. In other words, what people mean when they say "write what you know", if they're not just repeating cant, is that one should closely observe the world around them and express that through writing. A writer may not "know" objectively what it's like to hunt pirates in Indonesia, but if he can apply related experiences well and combine it with his own general observations about the world and how things and people move in it, he can give a fairly convincing account in between the facts he needs to research.
A rejection of this philosophy is a rejection of the fiction tradition, writing, art in general; in short its a rejection of the cumulative efforts of civilization for the last several thousand years. I think it's no accident that a common thing you hear these days is "you can't know if you weren't there". I reject this because to agree with it is to doom the world to the total destruction of all forms of storytelling through art, and in the end the destruction of the soul. I agree that first-hand knowledge is invaluable when writing about any topic. But what I don't agree with is that I can't use the power of my mind to imagine myself into a situation and get myself at least 75% there, enough for a good story. Again, there is more than one kind of knowledge. An all-too-common holding of this idea that "you had to be there" is why it seems people only read nonfiction these days (to look at the bestseller lists), for the most part. This is too bad.
That doesn't mean that a writer can't get it spectacularly wrong, of course. I think that comes from an over-reliance on bad information or facts and not enough storytelling. If the characters and the world are real enough, people will meet you halfway and maybe even believe that's how it really is. The select few who have, say, been to the North Pole may chuckle; let them write their own goddamn book.
This also ties into the idea of writing something "realistic". It's a common complaint of audiences that something or another that happens in a book wasn't "realistic". Sometimes this is true, like when an author gets his facts wrong. Most of the time, though, people are talking about the reality created by the work.
A writer has to be conscious of the fact that he is the God of the world he creates. He makes the rules, from the laws of physics and geometry to the people that inhabit it. He creates everything. Since men are not gods, this is why it makes sense to make the world as small as possible, so the writer can exact the control he needs. I've heard it said by some that when you write, there are no rules. This is incorrect, but I can see where the mistake is made. It's true that there's no rules in how a writer sets up his world. If he wants there to be werewolves, or aliens, or faster-than-light engines for starships, well then fiat lux and damn the torpedoes.
But, there are still some rules for good writing. The one I'm talking about comes down to consistency. The God of the book is not allowed to be mercurial. Now, if you want to create a God to run your universe for you and you want him to be mysterious, that's fine, since that's the rule you set up. But the author himself must follow his own rules. I know this is a little confusing - the point is that, again, this is a story, and the reader desires things to be resolved somehow, and for things to make sense. You can start your syllogism however you want, but it must follow the kind of logic that people understand. Characters cannot suddenly have a change of heart or personality. Things that worked before should work again. And so on.
Of course, for the story to have some surprises in it, you'll want to lead your reader down some blind alleys and have some twists. This is where the writing part comes in. You have to be able to bend your own rules, not break them. Maybe the character has been coming to this crossroads for a while and that's why he turned on his friends. Maybe the flu bug is evolving and growing resistant to the antivirals your scientist hero created. This is one of the very trickiest parts of a good story - get it right and people will be blown away and love the story. Get it wrong and "that just wasn't realistic".
Some people may not be able to get this, I think; that's why writers get letters about how it's simply impossible that a car should be able to jump over a moving train or whatever. I think listening too closely to these kinds of critiques is a mistake. It's one thing to get the facts wrong - if that car in your story has a 454 in it, but it shouldn't be able to take a 454, then the smart writer takes that into account, but he doesn't let it change his story or make his plot impossible. Otherwise there would be no point to writing. So some research is necessary; no one's saying it's not. The point is that the most important part of the story is the essential truth within it.
The writer has to be a student of human nature. The ability to recognize truth is built into all of us. The writer has to recognize that and be able to play with it, creating a world that's "realistic" no matter what crazy things are going on in it. Every person you meet, every thing you see, has a story, has material you can use. I think a writer can't afford to sleepwalk through life like so many people do. So pay attention, even though it makes you tired, and reap the rewards when you sit down and get cracking on the story.