Dialogue ought to be written by your characters. If you’ve got good characters, they will say what they think and all you really need to do is listen to them and transcribe what they say. Once you’ve got the basic dialogue down, you can polish it up a bit, adding descriptions sparingly and only as necessary to avoid possible confusion.
But keep in mind that a conversation can easily turn in directions you don’t want it to go and in doing so it can take a novel off course. This is where characters are rather like actors. They respond to the other person spontaneously and in character, but there’s nothing wrong with calling “Stop! We’re getting off topic here. Go back. Take it again from ….”
Nor is there anything wrong with knowing what the outcome of a dialogue ought to be. It’s legitimate to start a conversation with the knowledge “this is where x and y clash so violently that they say things they will later regret.” Or “this is where x learns about y’s vulnerabilities.” If you have developed good characters, you will only need put this out there and they will give you the actual lines – absolutely in character.
Characters talking with one another is, furthermore, far more effective that any narrator talking about what characters think or feel. Letting the characters speak for themselves is much more entertaining, authentic and exciting. After all, dialogue alone make a play, and theater is an older, arguably more effective, art form than novels.