Good dialogue:

At Life the Universe and Everything symposium, there was a panel on good dialog. Any inaccuracies or confusion, it is the fault of the note taker.

Take out all the unnecessary and as and also cut the grunts.
People speak in fragments. Look for the core message and trim out subject and verb.
A character who’s highborn might speak formally.
Don’t have all the characters sound the same.
Read your dialogue out loud
Instead of said have a moment of character action. People do a large portion of nonverbal medication and reaction.
Instead of detailed response cut down. Example a character’s smile may be more sinister.
Dialogue less rather than more. If you have a page of dialogue cut down to a paragraph
Characters never say what they mean. There is a movie where character is going to perform a dangerous activity. The wife instead of saying “please be careful and survive” she says, “We need milk.”
Everyone has their own agenda. In dialogue they want you to stop talking so they can speak.
People who agree are boring. People who disagree are interesting. History between characters can provide conflict. Look at the nine characters in of the Lord of the rings.
Different people speak using their own tone, word choice, actions and their agenda behind what they say. For example in the movie store wars one person may want to fight the Empire, Another one doesn’t or they may have an alternative plan to resist other than destroy the death star.
We are creating an illusion of reality. Our dialogue needs to be entertaining and interesting.
There may be a phrase a character uses a lot. It is their personal phrase for example the author Brandon Sanders starts much of his dialogue with YA. Be careful of overuse for they can also be annoying.
In the movie bridge of spies, the Russian uses the phrase would it help. If he did a clearing of a throat that could become annoying.
Side characters may be a good way to present annoying attributes as they are not as frequent.
Watch a TV show or movie with no sound. And see what you can learn from their behavior or watch to people’s actions at a restaurant.
Listen to those around you while you’re eating out. Observe conversations and body language. Seek out what is interesting and unique.
Populate your life with people who have interesting stories. To do so, you must make yourself compassionate and generally interested in others. You will find magic in them.
Be careful of fancy dialogue that sounds modern