I can't generate a useful definition for good rhythm, but like the supreme court justice said about defining pornography, I know it when I see it.
There are certain very low-level mechanic aspects of writing dialog that can make a good difference, though.
Attribution Styles
How do you attach a given piece of dialog to a given character?
There are, mechanically, a handful of methods.
Basic Tag
"I know," he said.
He said, "I know." This one seems to have fallen into disfavor. It feels anachronistic.
"I have no idea," Danny said. "But I'll let you know when I find out."
"I have no idea, but I'll let you know when I find out," Danny said.
I prefer the first of the two above. It's a common mistake to push the tag too late into the character's dialog. We need to know quickly, for both rhythm and understanding, who's speaking.
If you're going to use this style of tag, you are, for the most part, best off sticking to 'said' and 'asked.' There are obvious times, liked a shouted sentence, that you should say they shouted. But don't get much more creative than that. Said and asked have the added benefit of being essentially invisible to most readers.
Action Tag
Here's an action tag--rather than the basic he-said, she-said approach. It tends to be closer to invisible (a very good thing) than those tags are.
Danny winced and shook his head. "I have no idea, but I'll let you know when I find out."
"I have no idea." Danny shook his head. "But I'll let you know when I find out."
No Tag
There's currently a strong preference for tagless runs of dialog. Readers like for pages to move, and it works, as a sort of artificial trick, to make the story feel like it's moving along, as well.
Carla stood nearby, simply staring. "Why?"
"I like glue."
"Again, I'll ask you, Why?"
"Curls my toes. And my nose hairs."
The risk, in long runs of this kind of dialog, is that it can feel very thin. It's also easy to write scenes that have no rhythm save an unchecked gallop.
Tid-Bits
* End with a comma if the tag is a continuation of the sentence, a period otherwise.
- "I hate it when you do that," Janey said.
- "I hate it when you do that." Janey turned an angry look his way.
* Get the rhythm of the tag's placement right:
"Why in God's name are you sniffing glue?" Carla asked.
"I like glue, because of the way it curls my toes and my nose hairs," Nick said.
In the previous sentence, the tag comes too late (though this is not a terrible example).
Instead, something like the following would work better.
"I like glue," Nick said. "Because of the way it curls my toes and my nose hairs."
This is an improvement because, most often, it's best to finish with dialog, rather than a tag. And, in this case, the tag is in the best place rhythmically.
* Forget the editorial
"I love you with all my heart," she said lovingly.
This is obviously intentionally bad. But the point is, either the dialog carries the tone or it doesn't. Your editorial only gets in the way. If the dialog hasn't carried the intended tone, it's too late for your editorial to help. I've already read it and, if I'm paying attention, had it ring in my ears in my own way.
If the dialog doesn't say what you'd intended, in the way you'd intended, the editorial aspect of the tag is unlikely to fix the problem.
I'll paraphrase John Gardner here--though I think he was quoting someone else--and say that writing should create a vivid and continuous dream in the reader's mind. When you're dealing with the mechanics of dialog, do your best to make those mechanics invisible, to avoid interruption of that vivid and continuous dream.
I'll refine this article and add a few thoughts soon, but I thought there was enough here to be worth putting out.
