The rant on genius characters, which has to do with ways of keeping them in check and having fun with them and using them in plots and…well, everything else I could think of, really.

1) Choose your intelligence.

The quickest way to make a genius character unbelievable is to make her a genius at everything, from music to magic to math to matters military, from poetry to pottery to planting to planning. Everything she turns her hand to is effortless for her. As a result, she solves all the plot problems, and becomes boring to read about. I’ve read short stories, and even sections of novels, before that are little more than recitations of the genius character’s amazing abilities. The author is stupefied with her creation, and I’m stupefied that anyone is actually content to write like that. (For more reason than one. See point 6).
So. Choose your kind of intelligence. It doesn’t have to depend on Western divisions of it. Perhaps the connections between music and mathematics in your world are open and explicit, and so it’s not astonishing that a genius at one is a deft hand with the other. Perhaps your military genius is good at both strategy and tactics. Perhaps magic in your world absolutely depends on knowing the movements of the stars, and so an expert astronomer is also an astonishing mage by default.
But don’t make it all-encompassing. The more specialized knowledge that your character has, in fact, the more years she’s spent steeped in one subject or another, the less likely it is that she’ll also be deeply acquainted with all the subjects remotely related to it, never mind subjects that have no relation. There’s the problems of time and acquiring knowledge, especially in a fantasy world (see point 2), and the problem of interest, and the problem of the plot. I would encourage you to go for specialized geniuses, in fact. Then neither the readers nor the author has to worry about this character solving every obstacle that happens along.

2) Genius != knowledge.

The genius astronomer-mage I talked about is not going to astonish anyone if he doesn’t know the movements of the stars, what comets and planets mean in this system, what the constellations are, what the best way to observe distant stars is, how to translate the movements into magic, and so on. Having him elaborate a system all on his own, and having it turn out to be the match or equal of the system already in place, is stretching plausibility very far. Many people think about genius scientists in our own world making astonishing discoveries, and assume they did it all on their own. The point is that those scientists didn’t start out in the desert of ignorance; they knew the basics of their own systems, what other scientists had discovered before them, and what conditions would produce verifiable results rather than ones that couldn’t be duplicated. It’s much easier to build genius out of a community, especially a community that can recognize your ability and name it for what it is (see point 3).
So. Don’t assume that “instinctive talent” will get your genius character everywhere. Show what kind of education he had, what he knew before he started, and what innovations he’s made. Find some way, if your fantasy world does not have a printing press, to account for where he might have read the books he’d need. If his genius is entirely based on practical experience, think about whom he could have learned from, and where he’d have come into contact with them. Utterly isolated backwater villages are not going to help you in this respect, as you’ll have to come up with a pretty convincing reason that there’s a master swordsman hanging around in one of them who just happens to recognize the budding genius’s talent and dedicate himself to training it. Cities and academic environments are much easier.
They’re easier for getting your character recognized, too.

3) Think about the context your genius is put in.

A modern Western audience would know it’s astonishing for a young poet to begin writing sonnets the equal of Shakespeare’s at 16. However, if the young poet lives in a twelfth-century village where there are no sonneteers, I want to know why they’d recognize what she’s doing as astonishing at all. They’d probably shrug, especially if the sonnets are excessively formal or use slant rhymes that are hard to remember. Her sonnets might get a singing around the fire, but it’s a stretch that the village would band together and raise thousands of gold coins to send her off to the local equivalent of Oxford. Prejudices against youth, gender, race, and wasting time with scribbling are going to weigh into that, never mind the fact that writing material in such an environment will be rare.
Don’t rely on the fact that your character might well be seen as a genius in the world outside your story. That persuades only your readers, and not all of them with any reliability. The characters in the story are going to have recognize it, too, and not by some silly “somehow she knew that this was remarkable, that she would never hear sonnets the equivalent of this” mechanism.
Alternatively, a genius sonneteer in a poetic family may be first among equals, but not cowered to or built up into the equivalent of Shakespeare. She’s exercising a talent that they recognize and honor her for, not something entirely new and eye-popping. She’s probably also had education in sonnet forms and by reading the sonnets of other poets, so they know she’s not producing it out of thin air.
The sonneteer in a temple who grows up learning in the temple school and composing sonnets in praise of her goddess is something else again. Perhaps all the formal, written prayers before that were in ottava rima The priestesses might wonder, might compare the sonnets with the old prayers and nod, might resist adopting them and start an academic and theological schism over whether the new prayers or the older ones are better. But there, as well as recognizing her poetic genius, the priestesses would have the concerns of theology to weigh them against, and whether or not these new poems honored the goddess.
There’s a rich supply of scenarios here. I named these because I like all of them better than the usual scenario where the genius gets “discovered,” brought into an environment where people practice the same skill, astonishes them all (and turns the evil ones bitter with envy), and ends up triumphing because she’s so obviously better. It’s too much a retread of our own Western concerns, and the myth that genius springs from nothing and flourishes like a rose in the world, and if it’s ignored or competed with, that’s a crime. It’s also, in some cases, very obviously the author using the book as therapy. The further she can travel from her own high school or college experience when she writes a genius in another world, the happier the audience is ultimately going to be.

4) Give the genius flaws related to his intelligence that are not absent-mindedness or social ineptness.

Those are clichés by now—the absent-minded professor, the character who lives in a medieval fantasy world but might as well have “NERD” typed on his forehead. There are plenty of other flaws related to intelligence that you can use instead.
One is lack of empathy. The person who believes that what she can do isn’t astonishing, that everyone could do the same thing if they concentrated only a little bit, is annoying to many. (I’ve been in classes with people like that; they make terrible teachers and students, both). She’ll probably get impatient easily and stalk away from someone who needs to be shown the skill just a few more times. And if you’ve got a character who’s a certifiable genius, the likelihood will increase. She can see the way that this magical pattern needs to be held in the mind, why can’t everyone else? They must be stupid!
So, of course, close beside lack of empathy runs arrogance. I don’t think it’s a contradiction for someone to believe that what she can do isn’t that special and yet act with cool hauteur to everyone around her. She sees herself as practicing a modest skill, and these dunderheads won’t stand up and take notice that it’s a modest skill. Argh! How frustrating! Such a character practically writes herself, flaws and all. The main challenge becomes for the author not to get so caught up in her vision of reality that he starts writing all the other characters as truly stupid.
There’s also obsession. I’m sure you’ve had the experience of getting cornered by someone who keeps chattering on about a subject in which you have no interest. Write a genius who does that, and then, while you can still make him socially inept, the focus moves off clichés like not knowing which fork to use at a formal dinner. The genius mage can forget that not everyone has the same compelling interest in magic that he does, and talk people who have even a nominal interest into the ground. Obsessions aren’t always pretty from the outside, however much they sparkle and gleam from the inside.
Just one more flaw, and I’ll move on to the next point: the tendency to rely on that one skill to the exclusion of all else. The mage with a great deal of specialized knowledge in water magic may become helpless when he gets kidnapped and taken far inland; that’s a given. But a genius mage who’s never studied anything else might not even try another route at escape. He’ll think that he has to use water magic, and keep trying long past the point when someone else would have started to sweet-talk the captors or get the ropes loose.
There is no reason that a genius character has to solve all the problems in your story. He’ll cause a great deal more if you just start looking at what the consequences of his knowledge are (and, of course, don’t make him good at everything under the sun).

5) Give him flaws that aren’t related to his intelligence.

This is short and sweet and probably obvious, but it sure gets ignored a lot when it comes to geniuses. That one trait grows and becomes not a part of them, but them. The character has no flaws that are not related to his genius. He has no knowledge, or lack thereof, that does not relate to it. The reactions of every character around him are shaped by their knowledge of him being brilliant in that one particular area.
This is the road back to a stock character, and you’ll want to avoid it. There is no absolute law that says genius characters can’t be hypocrites, gossips, sulky, quick-tempered, prone to jumping to conclusions, irrational about not getting their favorite food at dinner one night, or believers in stupid superstitions. I would find it more refreshing if more of them were.
So. Spin a complete person, not just “a genius.” It would help.

6) SHOW.

Genius is one of those qualities that, for me, must be shown and not just told. The writer might tell me that a character’s tall, and I’ll nod and accept it. The writer might tell me that he’s handsome, and I’ll shrug—maybe wonder why no one along the way reacts to him as if he were handsome, but not worry greatly about it. The writer might tell me that the character’s a genius at poetry, and then the first atrociously rhymed poem which doesn’t scan comes along and I’ll laugh my ass off.
You commit yourself to doing a genius, you commit yourself to demonstrating his genius. This might be by indirect description rather than actual transcription, especially if you yourself do not share the talent. You might talk about the effect that a political orator’s speech has on his audience, for example, rather than writing it out. But that’s not always going to be an escape, and if you do it too often, it’s obviously an escape instead of a writing choice. Sooner or later the military genius will have to march into battle, the genius mage will have to do something awesome and clever with his magic, and the master swordsman will have to fight.
This is where those sections of short stories and novels that are simply lists of a character’s amazing abilities—“Oh, she can read more languages than anyone alive and fling firebolts hot enough to melt steel and see the future and the past in her dreams and her mother was the last werewolf”—are not just clunky or stupefying, but dismal failures. The author tells them, but does not show them. The character often doesn’t read languages at all, or the description of her doing so is childish. The author gives the audience something utterly prosaic, not astonishing, claims it’s astonishing, and expects the readers to cheer.
Your characters have to be more than words on a page to engage the reader’s heart and imagination. They have to be more than statements of genius or abilities in order to look smart or skilled. And if you tell your readers that you have someone who can do wonderful things, guess what? You’ve just committed yourself to giving them someone and something wonderful.
Good luck.

7) Chance says, “Fuck you,” to genius as often as to anything else.

A master swordswoman fighting on ice? Who’s to say that the ice won’t crack beneath her and take her down into the dark waters? And, while I have no doubt that she’s skilled with the sword, I have my doubts that she’s similarly skilled at fighting hypothermia.
The scientific genius goes on a journey to study animals and concoct his astonishing theories about them? There’s no saying that an insect won’t bite him and inflict him with a crippling disease. Insects don’t generally care whether they’re biting geniuses or not.
The world explorer goes sailing off on a journey around the world and brings back valuable knowledge? That doesn’t stop a plague from striking at home, or civil war. He may not have a home to come back to.
I bring this up because, often, when something does befall a genius character, it’s always, always connected back to the fact that he’s a genius, Point 4 all over again. The master swordswoman knows about the ice, fights there deliberately, and manages to swim to shore before the cold can take her (often with the author obviously having no idea how quickly winter water kills, or how much armor weighs). The scientific genius gets ignored and slighted by his jealous contemporaries. The sailor comes home to find that his wife gave up on him and married someone else. They should be pitied, because their decisions in the name of genius are what recoil back on them.
Don’t always do this. Be aware of what traps are going to lie in your world whether or not your character is skilled at dealing with them. Make this person a citizen of your world, once more, not excepted from misfortunes that would take down anyone else because of what they are.
Geniuses are damn fun to write, and possibly wonderful fun to plot with, but too many become their intelligence or talent instead of intelligent and talented people.