One of the many areas that some writers find problematic about dialog is the use of idioms. This is especially tricky for SF and fantasy writers who are trying to create a realistic-sounding but still-comprehensible imaginary world.
The first common mistake, especially for science fiction writers, is to go to one extreme or the other – either the writer unthinkingly uses nothing but current real-life idioms and catchphrases, like “once in a blue moon,” or the writer uses nothing but their own made-up idioms.
If you look at common, ordinary American speech in 2012, you will find common phrases and idioms that arose long ago, like “keep it under tight rein” or “give it free rein”…but you will also find ones of much more recent vintage, like “I don’t have the bandwidth to do that today.” A future society that uses only those idioms and catchphrases that are currently in use in English implies that nobody has invented a catchy new turn of phrase in the intervening time, that none of the catchphrases or idioms in use in other cultures will migrate into the English language, and that none of the current idioms will shift in meaning. This is unlikely, to say the least.
The second problem is that while people occasionally use clichés, idioms, and other such turns of phrase in their conversations, most of us don’t use them constantly. And a novel gives the impression of things happening very quickly. It is very likely that in real life someone would say “time flies” to one person and then, a couple of days later, “add his two cents” to another conversation. In a novel, those two conversations, days apart, can quite easily take place on consecutive pages. This gives the illusion that the characters are talking in clichés all the time, even when they aren’t. And a conversation like the following would be ridiculous: “Harry! Haven’t seen you in a blue moon.” “It has been a coon’s age, hasn’t it? How’s your aunt?” “Fit as a fiddle. Her son is always in trouble though, and keeps leaving her holding the bag.” “And him born with a silver spoon in his mouth! What’s the world coming to?” “You hadn’t heard? I thought it was the talk of the town.”
The third problem is that realistic dialog in a novel is not a transcript of the way people actually talk. If you were to tape-record a conversation on a bus, or over dinner, and then transcribe it exactly, you would have a lot of boring, unrealistic-reading stuff like “Well, you know, it was, um, that other one – yeah, the pink, and the thing, er, went…I dunno, it, um, went. So I said, like, um, wow, I mean really. But, um, then it stopped.”
Dialog in novels is a model of the way people speak in real life. It leaves out all the “ums” and “ers” and “likes” and “I means” and the rest of the verbal static that people use in real-life conversations. Dialog also tends to be a lot more coherent – you don’t see as many sentence fragments or dangling bits as you do in real-life conversations. And you don’t see as many idioms.
There are several ways of getting around these problems. One author had a modern-day character moving around in time; the people he talked to expressed amazement at his cleverness and wit every time he used a phrase like “time flies when you’re having fun,” because they’d never heard it before. Several books have postulated a future in which different social groups were based on different past eras, and tried to use the language, dress, and social customs of the time period they were re-creating (rather like the Society for Creative Anachronism, or some of the Civil War re-creation groups).
A more common solution is for the writer to invent his/her own idioms, ones that would arise out of the kind of society (different from ours) that the characters live in. In our society, “too many cooks spoil the broth;” in a seafaring or spacefaring culture it might be “you can’t have two captains on a ship;” in a culture of traveling merchants, it could be “the more people involved in bargaining, the worse the deal.” An excellent idea-generator for this sort of thing is IDIOM’S DELIGHT, by Suzanne Brock, which lists a whole bunch of Spanish, French, Italian, and Latin phrases that are just as commonplace and “clichéd” in those languages as ours are in English. (My favorite is Italian – instead of “I see you once in a blue moon,” they say “I see you once every death of a pope.”)