I was reading a book the other day by a new-to-me author. The premise sounded interesting and the plot was doing a fair job of holding my interest. Unfortunately, I had to DNF the book. The dialogue... OMG, the dialogue... It was bad. Like an example of bad dialogue in a 'how to write' book. People don't talk to each other that way and after a while, I had to close the book to save my sanity.
Then I was working on the edits for Blink of an I - a novel I initially wrote early in my career. And OMG, the dialogue! It was bad. I could hear the characters talking to each other in my head and realizing people don't talk to each other that way - not even way in the future after the world has fallen apart. And I swear I must've used every dialogue tag in the Big Book of Dialogue Tags. 'He exclaimed', 'she hissed', 'he sneered', 'she whispered', 'he choked'... And I was all like 'who wrote this crap... oh, it was me'.
So, when I noticed this yesterday morning, I went back and re-edited the 45 pages I had already done. So bad. So much dreck. Bleh. It took me all day, but it's fixed now. And I'll scour the remaining pages to make sure as I edit along, I fix what seems bad.
Anyway, if you aren't already reading your dialogue out loud, do so. If it sounds unnatural to you, then it will sound even worse in your readers' heads.
"But, but, but," you say, "my book is set in Victorian England (or 2351AD or a million years BC or on Tahiti or in Jamaica or wherever/whenever) and they talk different." Sure, they do. But you'll find that no matter where or when your characters are, there will be a rhythm to their speech and there will be things they'll say or won't say, or natural ways they'll talk that will fit and feel right to your readers.
Always work toward dialogue that will feel right to your readers. The goal is to keep them reading, not to make them DNF a book for any reason.
So, anyway, Blink is on its way to being fixed. There's a lot of work ahead and I still have life stuff I need to attend to, so I'd better get cracking. Have an awesome weekend, and don't forget to watch that dialogue. ;o)