Casualness. Familiarity. It's basically show don't tell taken to a whole new level. This, I think, is what people love about Harry Potter -- the extraordinary settings are fleshed out perfectly by the way that it's all normal to the people who live there.
Since stories usually start with change, with the character being thrust into an unfamiliar situation, we often focus on how to write this, how to explain and describe everything and the character's reactions to it and the new stuff/people's reactions to the character. But what we forget is that what doesn't surprise a character can tell as as much as what does.
I love it when a science fiction or fantasy novel starts with cool stuff happening that's commonplace then and/or there. And it's amazing how quickly new technology becomes commonplace. Take mobiles (cell phones to US people). Thirty years ago, people in sci-fi had these cool things (sometimes called comms), that could communicate like a normal phone, but wirelessly from anywhere.
[I can feel people clicking BACK right now, thinking "Well, I don't write fantasy/sci-fi". Don't! Everyone, whether they're in the present, the past, the future, or an alternate dimension, has their own "normal". Even if your target audience is the same age, gender, time and interest group as the character, it's important to realistically portray what's normal for them, in fact, maybe more important than ever, because the target audience will be matching up the character's casualness level about it with their own. If your piano-playing character describes his piano like he's seeing it for the first time, it won't ring true with music-loving readers who rarely give a second thought to these minor details of things that are familiar to them.]
Unless the whole plot is that super-portable phones are changing the world, it's got to be a thing on the side. If we were writing a novel about today, it would be out of character for a character to have a whole paragraph, or even a sentence, describing what mobile phones are. So why would a character in a sci-fi need to describe the teleporter they use to get to work every day! A character in fantasy probably wouldn't be any more overexcited about their cleaning spell then you would be about a bottle of furniture polish. If they are, it's unrealistic and simply detracts from the atmosphere of the novel.
Of course, characters unfamiliar with the setting would be surprised by these things, like Harry Potter is amazed at the wizarding world when he first sees it. But then there are things that Harry's familiar with that people like the Weasleys aren't -- phones, computers, elecrticity -- and the fact that this stuff, which is familiar to us, amazes them is as important as the magical stuff intriguing Harry. Everone is familiar with something, and showing what their "normal" is deepens the character and allows the reader to understand their reactions to the changes.
So how to we make familiarity?
1. Not noticing the stuff. What the character doesn't notice. In novels, especially those written in first or close third person, it can be hard to have something that the character knows, but the reader doesn't. In a movie, they can walk past a super huge dragon thing in a zoo without caring, but in a book, description of something suggests that its existence has been acknowledged.
Mostly you have to work this out yourself according to context. Sometimes it's obvious -- a character who's a dancer might just say something like "after dancing I..." or "at dancing Megan said...", where the reference to dancing is simply a description of where she was when something happened. This shows that she's familiar with dancing -- it's normal for her and doesn't need more explaining. A character who goes to a Gifted class at school would just say, "I missed PE because I was at Extension Maths".
2. They don't complain about it. You might complain if your house roof was leaking, but someone living in a tent would probably say, "I almost tripped over the rainwater bucket".
3. They do complain about it. New and future technology is usually viewed as pretty good. People thirty years ago might have thought a world with computers would be great. Well, 1970s people obviously didn't just try to comment on an author's blog which had about a three-second typing delay that made backspacing super annoying because I accidentally deleted most of my comment. Or spend hours at computer fixing people... or back up the thing they were writing because they know the stupid thing could crash any minute.
See where we can go with this? Kid's annoyed he missed PE for Extension Maths. Stupid mobile phone rings in the middle of class and gets you in trouble. Ursula had her outfit all worked out for Caitlin's party, but then she had to go to dance rehearsal. The complaint takes the focus away from the unusualness of the situation (to the reader), at the same time as emphasising the fact that the person is familiar enough to complain about the technology.
4. There are a range of responses. The breakdown of society might be bad for adults, but hey, for your fifteen year old protag it means NO MORE STUPID PHYSICS EXAMS! In the real world, people will find good and bad features in all situations, and fiction needs to reflect that. The quickest way to make something unrealistic is if everybody like it, or everybody hates it. Some people are love it, some like it, some hate it, some people just don't care.
5. Slang. This almost speaks for itself. We shorten the word "cellular telephone" because we (well, people in the US) use it a lot, in informal situations. Most people don't shorten "microprocessor" because we don't use it much. We feel comfortable renaming what's familiar to us.
6. They talk about what is new. A realistic world is always changing (unless part of the plot is that the world is stuck in a technological and creative rut, which is an interesting premise to explore). Remember, the future has a future, and the past has a future too. If a character mentions a hovercar with some new feature, this tells the reader that hovercars exist, they're probably pretty commonplace, the kind of thing people talk about.
Sample conversation, 2010:
Ethan: Did you hear on the news 3D TVs are going to be released, like, this July?
Jack: 3D movies give my mum headaches.
What this tells us: In this time, 3D technology exists, people have had some contact with 3D technology, but it's enough of a novelty to talk about, there are a range of responses to it (Ethan: excited, Jack's mum: dread), and most importantly, something else existed before it -- 3D television being a novelty suggests that normal television isn't.
Sample conversation, 2100:
Blaze: Crap, man! My apartment block's getting a force-floor next week.
Xac: What's crap about that? Our stupid block hasn't even got a force lift yet.
Blaze: Part of it's gonna be right outside my apartment and the kids next door are already planning how they're going to play on it. All night, probably. Plus, that nice woman up the hall -- you know the one who's been there for ages and cooks stuff by hand -- she's so freaked out by the idea of walking along "invisible air stuff", she's planning to move out, so probably more kids'll move into her place.
what this tells us: There's something called a force-floor, which seems pretty new. A force-lift, however, is apparently more common, almost considered a necessity by Xac. An old lady cooks "by hand" -- the fact that this is mentioned suggests that cooking is an uncommon skill. Some people are scared by the new technology. Finally, the new technology is described simply as "invisible air stuff" -- combined with "floor", we've got a pretty good impression of what this thing actually is (at least, I think? Is everyone else imagining a transparent floor, several storeys up? I'm with the freaked out lady on that idea).
That about takes care of why and how we do it, with one exception. What if your character is referring to something nobody understands? Like a bit of technology and someone's got to explain what it does?
1. The classic way to overcome this is to have an experienced character explain it to a less knowledgeable character. That way it also gets explains to the audience without seeming infodump-y.
2. If it's something fairly similar to something now, why now just use the same word? Houses now look different to houses 500 years ago, but they're still called houses. Just because a car in year 2100 is glow-in-the-dark, spherical, and powered by bacteria, doesn't mean it wouldn't still be called a car. Changes to technology come about gradually. If we saw a 1950s computer and then the laptop I'm typing on now, we might not say they were the same thing, but as the technology improved gradually, each new stage was just called a "computer".
3. Slang works well -- as things become more common, we use slang for them. Scott Westerfeld has made up some amazing slang words for his novels, and explains how to come up with the words in Bogus to Bubbly (which I recommend even if you're not fans of the series -- it's a great example of the kind of research needed to flesh out a science-fiction world). But let's focus on how to explain what these slang words mean. One way to do this is to make it so the "real" (non-slang) word for a technology or magic thing is a decent explanation of what it actually is.
For example, use "telepathic telephone", then use this in close proximity to the slang -- "teltel" so the reader understands that a teltel is a telepathic telephone. In real life, most real names indicate what a thing is. Mobile phone = portable (mobile) telephone. Computer = thing that computes (makes calculations), which is actually all computers do, but in fact doesn't really explain their use, as we primarily use them for stuff other than maths. It's usually not hard to find an excuse to use the "real" word for a thing, whether the character is talking to an older person who's not as into slang as them, or it's just one of those things where you sometimes use the slang word and sometimes the real name.
Another thing: do you need to explain it? I give you Kibble's Seventh Law of Familiarity:
7. We don't understand the things that are most familiar to us. We know how to make a computer do what we want (well, to a point), and how to drive a car. We can also describe the media- or school-explained version of how some new, exciting stuff like solar panels work.
Your science fiction character can drive a spaceship without knowing how rocket engines work. Harry Potter can do a spell without knowing how his wand works. You don't need to be able to make a piano to play one. A reader might get a more realistic impression of a character's world without unnecessary descriptions of how everything was made and came to be. To most people, the truly familiar stuff just isn't interesting enough to understand.
