In their excellent book, Self-Editing for Fiction Writers, Renni Browne and Dave King refer to an acronym that they say editors frequently write in the margins of manuscripts while editing: R.U.E., Resist the Urge to Explain.
The need to explain writers often feel is a classic case of a "show, don't tell" violation. Explanations always tell, and are especially noticeable in dialogue. A typical case is a writer's desire to convey emotion in dialogue.
As Julie Daines explained in an earlier post, as soon as you name the emotion your character is feeling, you stop showing and tell. Telling an emotion removes the reader's opportunity to feel it along with the character.
In dialogue, writers often explain a character's feelings through adverbs. For example:
Bob walked into the room. Dave sat on the couch. He looked depressed.
"Come on," Bob said. You need to do something fun."
Dave looked at Bob. "I don't feel like doing anything today," he said listlessly.
Both he looked depressed and he said listlessly explain how Dave is feeling, but right now I want to concentrate on that he said listlessly. We often do this because we lack confidence in our ability to make the reader recognize what the character is feeling.
Fortunately, this is a relatively easy problem to fix. Scan your manuscript for adverbs and mark every one. Then, go back and look at each adverb and examine what it is doing. If the adverb explains something that can be shown in the context of the story (and it frequently will), strike it and, if necessary beef up the text.
Bob walked into the room. Dave slouched on the couch, his legs draped over the arm rest and his arms flopping off the side.
"Come on," Bob said. "We've got to get you out of this funk."
Dave didn't move or look up at his friend. "I don't feel like doing anything today."
We get it from the context of the passage. Dave has no energy. Verbs like flopped, draped, and slouched, and his inability to move or even look up, combined with Bob's desire to get Dave out of his funk, all show us Dave is listless. Dave's lack of energy and the reason why he is listless are probably clear from surrounding text. If he said listlessly is the only sense we give the reader of how Dave is feeling, we haven't done our job. That explanation will not help the reader get a sense of Dave's state of mind. You could tack it on to the end of Dave's statement, but then it is still telling, and is redundant because you've always shown that Dave is in a funk and lacks energy. Trust yourself and trust your reader to get that from the context. Resist the urge to drive home the point you're trying to make by explaining it.
But adverbs aren't the only R.U.E. problem that shows up in dialogue. We often throw in a verb to explain our intent.
"Junior! Don't play in the street," Lisa yelled.
"Shh. I think there's somebody in the house," Mike whispered.
Whether your character yells, shouts, whispers, enthuses, or does anything else where you're asking your verb to explain the way a line is said, you are telling something that you should show. If your dialogue is written well, the reader automatically knows how the line is said.
Good dialogue always shows. After all, when you present dialogue, you are showing the action of a conversation, usually to move the plot forward like you would with any other action. So why weaken it by throwing in phrases of exposition, of telling? Especially when it's not necessary and all you're doing is explaining something you've already shown.