I just happened upon this over at Jon Winokur's Advice to Writers:
1. Dialogue should be brief.
2. It should add to the reader’s present knowledge.
3. It should eliminate the routine exchanges of ordinary conversation.
4. It should convey a sense of spontaneity but eliminate the repetitiveness of real talk.
5. It should keep the story moving forward.
6. It should be revelatory of the speaker’s character, both directly and indirectly.
7. It should show the relationships among people.
This advice is clearly for aspiring authors, but I figured it wouldn't hurt me to go down the checklist and make sure I've been staying on target in my own writing. I must say, though, that dialogue is the part of novel-writing that has always come easiest to me. Even as a beginner, I had no trouble with points number 3 and 4, which seem to be the ones that trip up almost every other inexperienced writer. I've had to work hard in other areas (description, for example), but I can write dialogue in my sleep, and that's probably why I got published. (I am not a strong plotter, but I can write a compelling conversation, which is a must for anyone submitting character-driven romances to the traditional publishing houses.)
If you're not sure what Bowen means by "routine exchanges of information" here's a hint:
"Jack, it's been ages! How have you been?"
"Just great. How about you?"
"I can't complain. How are the wife and kids?"
"The same. How are you liking that new job?"
"Lots of new challenges. I miss the old place, though."
"And we miss you, believe me."
All right, enough. I'm afraid I'll injure my brain if I force myself to write any more of that drivel. But you get it, right? Jack and the other guy used to work together and they haven't seen each other in a while, so now they're catching up. The above conversation reads like it was transcribed verbatim from a real-life exchange, and that is precisely what's wrong with it. It will not work in a novel because novels aren't about real-life conversations any more than Van Gogh's paintings are about sunflowers.
If you're writing dialogue today, you might want to bear in mind that suggestion is one of the most powerful tools a writer can wield. You don't have to write a full conversation at all. Just make your readers believe that you did. Here's an example:
He hadn't seen Jack in more than a year, not since he'd left his old accounting firm, Bean, Bean, and Bean. They exchanged a handshake and asked about each other's families, and then after a nervous glance around the coffee shop, Jack leaned forward and spoke in a low, urgent voice.
"What have you heard about the investigation?"
"All I can tell you, Jack, is that the SEC asked me a few questions. But we expected that, didn't we?"
"You didn't tell them about--"
"Of course I didn't tell them. I'm not a fool, Jack!"
See the difference? Hurry past the inconsequential stuff and get straight to the juicy parts. That'll keep your readers reading.