First up, the over-explanation, telling readers what they already know. It can happen in a range of ways, including single line statements-of-obvious. I'm focusing more on full explanations in dialogue. It's sort of the opposite of As-You-Know-Bob syndrome. In this case, Bob doesn't know the following information, but the reader does.
And it's really, really annoying to read.
There are times one character needs to explain to another what has happened, what the plan is, etc. I can only think of a few times this should happen "live" on the page.
- When revealing information previously withheld from the reader. I have a little of this in one of my novels, where I've only hinted at things, until the MC reveals her secrets later on. Hopefully (if I've pulled it off right), this kind of explanation is rewarding to the reader, verifying their guesses or giving some surprises.
- When the explain-ee's reaction is important to the plot. Is this information going to prompt a major event? Divorce filing? Attempted murder? Okay, maybe something a little less extreme could work, too.
- When the explain-ee will have new information to add. Maybe the reader already knows the MC's half of the story, but another character may have info to fill in gaps that change the whole outlook.
(Could have sworn I had a #4 in mind. Will add if I remember it.)
An important note: #1 is often the only time you might need to play out the full conversation. Many of these are situations where tell-don't-show is actually the best course. (I summarized everything we knew so far.)
In most other situations where it's necessary to fill in another character, there's one strategy I find particularly effective: the art of the skillful scene/chapter break.
Character 1: "We have a lot to talk about."
Character 2: "Say WHAT?" (or other appropriate reaction)
Can you think of other situations where playing out information the reader already knows may be desirable? Do you have strategies for avoiding the for-Pete's-sake-we-already-know-this reaction from your readers?