I attend a writers group. Here are some thoughts about dialogue from the meeting.
The way people speak can reveal their education levels, origins (accents and colloquialisms), and personalities. You can get a sense of them without a lot of explicit description.
Use of italics, caps, and exclamation marks creates different effects in the readers’ minds.
When should you use things like ‘he shouted’ and ‘he cried’ instead of exclamation marks, caps, etc? Maybe just a matter of style, but for me, these feel less immediate, less direct.
It’s helpful to imagine people you know delivering your dialogue – this can help keep it realistic and flowing.
Transitions – abrupt topic changes happen in real conversations, but do they disrupt the flow of written dialogue too much? Should there be some sort of cue to alert the reader (ie. some thoughts, ‘anyway,’ shrug, etc)?
It is helpful to have someone else read your dialogue aloud to see if it sounds natural.
Since you wrote all the dialogues, there is a danger that all characters will sound the same. One way to avoid this is to give each character distinctly different interests, mannerisms, and speech patterns.
What do you do if you discover that some of your characters don’t say much? You could invent new speeches for them, transfer existing speeches from other characters, or cut the character completely.
Orson Scott Card (the author of Ender’s Game) faced these dilemmas in his book Speaker for the Dead. He originally had a large family with members that felt indistinguishable. He ‘fixed’ this by giving them distinctly different personalities and mannerisms. In the process, he merged a few characters together. Having a smaller set of strong characters may be better than a large cast of less memorable ones.