Janet Fitch writes about writing dialogue:
Dialogue is only for conflict.
It’s like a racehorse, it can’t just carry any old thing, the pots and pans and old tires. You can’t heap all your expository business on it, the meet and greet, all that yack. It’s just for the conflict between one character and another. That’s it.
So if characters agree, you don’t need dialogue! If someone’s just buying a donut, nobody needs to say anything. That’s what narrative is for.
Also, great dialogue in fiction isn’t screenplay. In fiction you can just tell us what people are thinking, they don’t need to say the obvious. In fact, the most interesting fictional dialogue has people thinking one thing and saying another. That’s what gives your scene dimension, and it’s super fun to do.
The question in dialogue is always, who wins and who loses. Who is putting pressure on who, and how.
Dialogue works best in short bursts, three or five lines, then go back into the other tools of writing–landscape, internal thought, memory, observation, gesture and so on.
Keep it short. People don’t generally speak in full sentences. And nobody gets to make a speech, unless it increases the tension of the scene–where I’m waiting to see if you’re going to get me on that plane and don’t dare interrupt your long story about your grandmother’s prize apple pie.
I’m not sure how I feel about the ‘only for conflict’ thing — for me to agree we’d have to broaden the definition of ‘conflict’ so far that it would wind up being useless — but the last bit, about keeping it short, addresses one of my principle irritations in literature. Especially old literature. I _hate_ speechifying, where characters who hate each other stand around and give two page speeches about how much they hate each other, or deliver ten minute orations, or whatever. Nobody does that; nobody has ever done that. If you want to do that, write something else. Write an essay. I don’t want that shit in my fiction any more than I want raisins in my chocolate chip cookies. Which doesn’t mean that it’s A Rule that you can’t do it; only that if you do it, it will suck, in the same way that if you try to write in a vernacular you don’t really know, that will suck.
I want to think about the “think one thing, say another” bit, though.