In my first draft, I write terrible, on the nose dialog. Characters say exactly what they are thinking and the purpose of the conversation is incredibly clear. In real life, of course, it never happens that way, but if I don’t start out there, especially with high stakes conversations (i.e. fights over the tops of Volvos), I write myself around in circles trying to figure out what I actually need out of the conversation.
So instead I begin with what I need from the conversation, and when I circle back around on the next round of edits, I use observed speech patterns to fragment and color it. I often write down interesting or specific turns of the phrase when I hear them, and I always read my dialog out loud in the final rounds.