Conversations between characters can elevate a good story into a great one. While poorly written dialog can easily ruin a great story. Good dialog has to fit the story, sound authentic to the characters, and leave the reader room to fill in the blanks. These are difficult ideas to grasp, sometimes even for experienced writers.
Dialog doesn’t have to be grammatically correct. Most people don’t communicate in complete sentences, especially not with close friends and family. Even in situations where proper grammar might be appropriate, people often speak in their personal shorthand.
Accents can be used to great effect, if they are internally consistent. Editors may hate you for using an accent, but if it fits the story, go for it. My only caution is that you should be intimately familiar with the accent your characters use. I’m very comfortable with Southern and Midwestern accents (A lot of people might think Midwesterners have NO accent). But I’d shy away from New England pronunciations.
Poor dialog can stem from the author trying to cram too much into what their characters are saying. Some examples (especially in Sci-Fi/Fantasy) include, “As you know Bob…”. If Bob already knew, you’re only saying it aloud for the reader, who might resent being spoon fed. Find other ways to share information.
Sometimes you may be tempted to include oversimplified logical deductions. Or you might be tempted to info-dump. Speech can allow you to share information, but that might be better spread through other means. Or leave it out all together and imply it through better methods.
The best dialog comes naturally, sounds like readers expect characters to speak, and leaves unspoken the volumes an author might be tempted to cram into a few short exchanges. Some of this comes down to how well you know your characters, but even with supporting characters, less is more. Don’t shy away from letting them speak, just be aware of how they sound (reading dialog aloud helps!).
One last suggestion about dialog. In addition to using the simplest dialog tags possible, don’t ignore the opportunity to fix your characters in space and show them in motion. Most of us don’t stand motionless as we speak. We sit up, lay down, walk around, and DO things while we speak. Your characters can do all of this, building on the descriptions of setting and action already part of your scenes. Like nutmeg, this is often an under-utilized spice among newer writers.
This is one of those topics that can make up a whole on-line class by itself. There are certainly enough e-books and how-to manuals on amazon and elsewhere. As long as the voices of your characters are authentic, you’ve started on the right path. You can embellish or cut out content later. Fix how characters should sound in your mind, and the rest will follow.