I love dialogue and complex stories, but I've been noticing most authors make what I consider basic mistakes by trying to jam too much into dialogue. This takes two most common forms.

In one form, they're so eager to "show, don't tell" that they insert background facts into dialogue that simply wouldn't be spoken. (Or, worse, they tend in this direction enough that a reader begins to hear every spoken factoid with suspicion.) They're afraid to say anything in the narration. They want everything spoken, even if it sounds unnatural and clunky. Let the narrator tell me it's the third bomb threat this week. When two people standing next to each other for all three threats exchange "That's the third bomb threat in one week!" it almost doesn't matter anymore whether it's realistic. It already sounds like bad exposition. There's no shame in narrating the awkward corners of dialogue if they probably wouldn't be said out loud in that moment.

In the other form, writers make the same kind of mistake with dialogue tags and interstitial action. They're again too eager to "show, don't tell." The author is so afraid to directly address how a character seems to another that everyone's constantly blanching, stammering, gawking, blushing, etc. In real life, most people rarely stammer, and when they do, it might mean nothing or not at all what you would suspect. Fiction gives us this illusion that involuntary activity is obvious, stereotypical, and reliable. Often it's none of those things, though it's always interesting.

"Show, don't tell" is great advice, but it isn't a hard and fast rule like "never drive head-on into an approaching truck." These errors I mention are hardly gigantic, but they'll break the spell, and then the writer and reader have to start over and cast it again together. Fortunately, this is avoidable.