There are some fundamental rules of dialogue:
1. Mimicking the way people actually speak as if you've recorded the voices of two or more people speaking doesn't work. When written, the way we speak rarely sounds authentic and often drags.
2. Make it clear who is speaking and that they are speaking. I don't require quotation marks because I think using italics or other devices can be effective, but readers have to know who's speaking so they don't get bumped from the world you've worked to create.
3. Don't use adverbs, he said firmly. And don't add modifiers, he exclaimed. The dialogue should convey all the emotion on its own. If you feel you need to add an adverb or modifier, then it means the dialogue doesn't work.
4. Less is more. Well selected words and phrases will carry more weight and do more work in service of story than rambling on and on.
5. When writing dialogue, see and feel from the character's point of view. This means you have to have empathy for them, even the jerks, in order to see the world and perceive it through their eyes.
6. Don't clutter it up with a bunch of crap. Keep the dialogue free of explanation and too many distractions such as stage directions to the reader. Here again, less is more. Also, think about human psychology. Someone lying turns their eyes down and to the right. It's a subtle move that conveys a lot of meaning. It doesn't have to be exaggerated or written in the equivalent of a neon sign so the reader doesn't miss it.
7. As with all things written: Show don't tell. For example, "It's ice cream!" the child said with the cute excitement particular to a small child when ice cream is coming. Versus: "It's ice cream!" she said flapping her hands.
Finally, get rid of excess words. There are always excess words. Every writer, no matter how great, has excess words in their writing. Find yours and get rid of them. Then ask someone who is an editor to go through and do the same. Every single word, phrase, sentence, paragraph, page, scene, chapter and section should carry as much weight as possible.