Levels of Diction
How do your narrator and your characters speak? What is their level of diction? In Thomas Pynchon's V., where ex-sailors hunt alligators in sewers (among other things), there is plenty of profanity, plus bawdy songs and naval slang. In the Evan Godolphin sections, the diction becomes quite elevated, befitting his background and education. In Mary Shelley's Frankenstein, the monster learns to speak by reading a few classic texts, so it stands to (okay, rather far-fetched) reason that he speaks formally, eloquently, correctly. It also serves as a bitter, poignant contrast to the horrors of his fate as an outsider and outcast from the society whose finer diction he has internalized.
Check over you dialogue to make sure your narrator or character would really use the words you associate with them. Are they too old, too young, too sophisticated? Run a plausibility check on your level of diction.