Writing engaging characters is no easy task. And while actions often speak louder than words, what rolls off a person’s tongue has its own distinct value. Not to mention ways to screw it up.
The most important gauge of good dialogue is believability. The words coming out of a character’s mouth should fit that unique personality and its background. If he’s a drunk working on a buzz, let him slur. If he’s from the Deep South and has an accent, show it. And if the character is young and inexperienced, make sure he sounds naive or a little ignorant compared to the more seasoned people around him. One of the best tests is for a writer to go through the manuscript and pick random lines of dialogue. If he can’t tell who the speaker is based on the line alone, it could probably use some strengthening.
Context also factors into believability. Readers know what other human beings sound like when they’re stressed, sad, or angry–just as they know how they would react themselves. A book’s characters need to be a reflection of that reality. Someone who just twisted his ankle and fell on the pavement is far less likely to say, “Ow! I think I sprained my ankle!” than a simple but forceful, “Fucking hell!” or “Goddammit!”
Consistency is also vital. There are few things more disconcerting to a reader than when a character suddenly alters a speech pattern that’s been present up to that point in the text. If a character’s slang or dialect alters a certain word, or kind of word, it should do so for the entirety of the story.
Contractions are also worth mentioning. Sometimes, writers use them far too infrequently in dialogue. This results in very stilted, mechanical deliveries that often sound ridiculous. Simply speaking the words out loud and listening for a reflexive contraction is a good way to check if one should be used or not.
These are just a few ways to ensure quality dialogue. As hard as writers strive to give each character a unique mind, it’s important to remember that the signals from that wonderful brain only have one tongue to work through and convey its impressions.