Here’s a post by James Scott Bell at Kill Zone Blog: The Curse of Expository Dialogue
Do not have characters reveal information that both characters already know. Here’s a ham-fisted example of what I mean:
“Sally! I didn’t expect to find you here at Central Market.”
“I often come here at lunchtime, Molly. Doing research for the senior partners at Dewey, Cheatham & Howe really creates an appetite.”
“Does your husband know his petite, thirty-year old wife enjoys greasy hamburgers?”
“Bill? What he doesn’t know won’t hurt him. Being a cop on the street, he has enough to worry about.“
This is, of course, good advice, which is why there is a term for this. As you probably know, this type of dialogue problem is referred to with the phrase, “As you know, Bob.”
Bell adds, “On the other hand, dialogue can be used to reveal information when the info is hidden within a tense exchange.“
That’s true too, certainly. Bell provides an example. However, I don’t believe the exchange needs to be at all tense; it just needs to be better written than as-you-know-Bob dialogue. This is not difficult, and you can do it without the characters necessarily saying much because their actions can speak for them.
I mean, you probably remember these three specific lines in Memory by LMB:
“Miles! Thank God you’re here.”
He heard Illyan’s voice, for a change more amiable than stressed. “Ivan, you idiot. What are you doing here?”
“Lady Alys!” His face softened. “What are you doing here?”
You don’t need prior knowledge of Ilyan and his relationships with all these people to get an immediate sense of each of those relationships. Rather than naked dialogue, you get just a bit of description. Bujod has such talent with movement tags in dialogue — or in this case, descriptive tags. His face softened. Poof, we’ve foreshadowed the relationship between Ilyan and Alys, even though such a thing was never previously suggested in the series.
Nor is there any need to worry about tension if you think about using expository dialogue in any situation where one of the characters is legitimately explaining something the other does not know. That’s one of the great advantages a naive protagonist gives you! Think of Moon in the Raksura series. One of the reasons Moon works so well as the protagonist is that he knows literally nothing about his own people, exactly as the reader knows nothing about them. There’s all kinds of expository dialogue and it’s not the least problem because (a) Martha Wells is a good writer, and (b) Moon needs a lot of explanations for a perfectly natural reason.
So honestly, I think picking on “expository writing” when you mean “this little subset of “as you know, Bob” expository writing is kind of unfair.
This made me look around for expository writing in a book of mine. I have this manuscript sitting here, so let me just skim the opening scene. … Okay, how about this:
He looked at me and then at one of his people who had come up beside him. He said to that man, “We must have pressed them even harder than we knew if they’ve left a tuyo for us. I suppose this must be the son of an important Ugaro lord, but he seems merely a boy.”
I must have jerked in outrage, for he turned quickly to look at me again. I said, speaking carefully in darau “Lord, I have nineteen winters, so I am not a boy either by your law or ours. You should accept me as tuyo. No one could set any fault against you for it.”
I think we get a lot of worldbuilding from this tiny snippet — information about both characters, too. There’s absolutely no need for either character to say, or think, “As you know, among both Ugaro and Lau, nineteen is considered adult.” There’s an implication that the age of adulthood might be different for the two peoples, but there’s no need to say so or explain what that difference might be. Those details are not important here. It’s quite clear that this “tuyo” concept is fraught. Later that term is spelled out more explicitly through dialogue. I don’t think anybody is going to find that exposition boring either:
“I’m not entirely familiar with the nuances of the tuyo custom. Sit up and look at me, Ryo inGara, and tell me: what is the usual manner of death for a tuyo?”
Exposition is not remotely off-limits in dialogue. If the dialogue is not working, that’s not because it’s being used as a vehicle for explanations or descriptions; it’s always bad for some other reason. The dialogue may be bad because it’s boring or stilted or artificial or unnecessary or clumsy or whatever. That problem, whatever it may be, should be addressed. If bad dialogue contains exposition, then both the dialogue and the exposition should be reconsidered.
But if the exposition works in the dialogue, then the reader won’t even notice it and certainly won’t be bothered. The dialogue will establish the characters and/or move the plot and/or contribute to the tension AND provide explanation or description, and the reader will zip right through it and be drawn straight through the scene without a hitch.