Between two characters on a page, there are certain things that need to be said. Knowing the words each will speak is one thing...putting them together in an order that feels natural is yet another.
One thing that really bothers me is reading a conversation that feels written. You know the type. Words nobody would ever say. Everyone speaking in complete and complicated sentences. Voices that feel same as the narrative voice. These are the obvious butcheries.
There's another more difficult obstacle to writing a good conversation and that's capturing the flow. When writing, there's this tendency to follow the and next, and next, and next model that keeps a story moving forward. But I find conversations rarely play out that way. There are pauses, subtleties, and tangents to consider. A conversation disrupts the natural course of the text. It requires special attention. And to make things even more complicated, the prose between the dialogue must be precise in order to hold up the and next structure of the story.
I've spent the last two days reconstructing pages of dialogue into a conversation. It's exhausting work. I hope readers appreciate the art of a good conversation.