When I'm writing a plotline, it often works out that I know two characters will be meeting each other in a particular scene, and that they will converse. And I oftentimes know the central idea of what needs to be communicated, and I see to it that it is.
But dialogue is more than that, and oftentimes I find myself wondering: "What else should they be saying?", or, more accurately, "What else would they be saying?" I find myself trying to flesh out a conversation without any particular goal in mind. I'm thinking, if these two people met, would they speak briefly and then part ways? If so, how soon would they part ways, and with what sort of farewells? Which one would be the one to say "All right, I've got to be somewhere."? Or would they hit it off and end up having a long conversation out of interest? What would they talk about? Or would one keep talking and the other wishes he would shut up? And if so, what would he be talking about?
Really, I think what I'm doing is aimlessly hoping that something will grow out of their interaction, that I'll suddenly realize I've stumbled upon a new idea for the story overall. It's happened before, but it's hit and miss.
How can I plan out conversations?