ETA: since I wrote this post I've developed a lot more thoughts on dialogue, though the problem I've unpicked here is still very common, and very easily solved!
A writer friend had feedback which said that her novel suffered from "ping-pong dialogue". Had any of us heard of this particular ailment, she asked here. None of us had, but the example she posted did suffer a bit from something I've seen a lot over the years, and no doubt I've been guilty of too; in fact, I'm rather grateful to have a name for it. It's not that the dialogue in itself is badly written; rather, it's a combination of things. Have a look at this:
"How long can you stay?" he asked.
"My bus doesn't go till six," she said.
She slung her jacket over the back of a chair.
"Would you like some coffee?"
"Only if you've got decaff, thanks."
"Yes, I've got some."
He put the kettle on to boil.
"You've cut back the hedge," she said.
"It got shredded in that storm. Had to do something," he replied.
Roly began to scratch at the back door and whine.
"I'll just let him out."
"Does he still come upstairs and bark if you stay in the bath too long?"
"No. He only did that to you." The kettle clicked off.
"No milk, please."
"Right you are."
He put the mug on the table.
"Did you get my letter?"
Now, there are various things going on here, all of which could be contributing to the ping-pong effect. First, it's not actually that briskly alternating lines of dialogue are a Bad Thing, but more that there isn't enough else going on. The dialogue has got detached from the other kinds of action in the scene: we've got speech-action, but not much physical action, or mental action and so on.
The effect, as someone else said on that WriteWords thread, is of "talking heads". In real life, we don't actually just sit there, immobile, and say things: speech is only part of the interaction of two characters: there's gesture and body-language: we move, or meet the other person's gaze or avoid it, or think one thing and say another, we have displacement activities like picking at fingernails or smoothing hair, we offer coffee by waving the coffee pot, or we decide not to offer coffee... Sometimes the other kinds of action are aligned with the speech: we shout "I hate you" and throw the saucepan. And sometimes they're in counterpoint: we concentrate hard on buttering the toast very neatly while we say, "Are you telling me you're having an affair?"
Even where there are actions in this dialogue, the writer hasn't put them on the same line as the dialogue by the same person, so the two kinds aren't connected even when they actually would be in the scene, and exacerbates the sense of talking heads. At the basic level, a good rule of thumb is to have something attached to every fifth line of dialogue, which shows who said it.
The other main thing that's going on in a passage like this is that every bit of the conversation has been written down, because that's what happened. It's a sort of narrative literalism, and is obviously a very natural outcome of how we imagine the story in the first place: to get at the right shape and pace of this scene, you probably need to imagine it in real time. But (as I was exploring in The Common Scaffold) that doesn't mean it should stay there in second draft. It's very easy to lose sight of whether, actually, each line is essential to taking the story (not necessarily plot) forward.
Working out what this scene is actually doing in the story can help you to decide which of all the little bits of dialogue are actually earning their keep. It's probably most useful to think in terms of compression, which can be used to focus the reader onto what's important. If my first example sounds a bit familiar to long-standing readers then you'd be right: it's what you could imagine as the first draft version of what I was using in Blow By Blow?, which might be your second draft:
"How long can you stay?' he asked.
She slung her jacket over the back of a chair. "My bus doesn't go till six."
"Good. I'll put the kettle on."
They sorted out the business of coffee - when had she gone decaff-only? - and he waited until the kettle had boiled and the dog been let out into the garden before he said, "Did you get my letter?"
If you click through to "Blow By Blow?" you'll find a discussion of why some of the apparently trivial bits (the decaff, the dog) got to stay in. But thinking in terms of compression and expansion isn't just so that you condense out the dull bits of what actually happened; it's about the rhythms of the passage.
In the first example the speech tags are all neatly tagged onto the end of the speech, which has a certain mechanical, binary effect - click-click-click. More generally, the first example lacks the weaving together of actions of speech, of gesture, of thinking which brings a much more flexible, varied rhythm to the writing. Among all its other uses, free indirect style and reported speech can break that on-off, binary, "talking heads" effect, by integrating what's said or thought into the rhythms and voice of the narrative. In the second example, even though it's a silly scrap written for demonstration purposes, that last sentence combines summary/telling, thought, physical action and direct speech, all into one. Because of that, it's much more energetic and forward-moving sentence, than if I'd separated them out into dialogue, speech tags and action.