"Emma? Dialogue isn't a swordfight."
Sigh. It's a metaphor. "As an author, it's really easy to get absorbed in the problem of moving the plot along. But –"
"Quick explanation, please."
"Think of dialogue as being like a swordfight."
Dialogue as a Swordfightby Emma Sohan
The main character probably wins the swordfight. That might be all that's needed for the plot. (Or perhaps he also gets injured, or gains confidence -- the point is, not much is needed for the plot.)
But no one writes a one-sentence swordfight. The swordfight is supposed to be interesting, or exciting, or generate some genre-appropriate emotion. We want surprise, twists, and suspense. Which is to say, the description of the swordfight advances the plot but also is interesting by itself.
Dialogue often does a lot to advance the plot, and there's no one-sentence shortening. But if you focus only on advancing the plot – what the dialogue is supposed to accomplish – the conversation tends to be dead.
To write a good conversation, you want to add genre-appropriate emotions. You want surprise, suspense, twists, and conflicts. You want interest and intrigue, or even humor. You probably want to show character. You don't need all of those in every conversation, but you want something.
How ToI should add, you don't want something completely unexpected in a swordfight, like your character suddenly having some new power or strength. It is the same for dialogue -- the unexpected should be within character.
Of course, that still leaves the problem of how to do avoid the expected and not be too unexpected. One way is if you can crawl inside your character's heads and let your character be natural. And again, focusing on the plot (and where you are going) or what you expect them to do doesn't work. Been there.
That's hard to do! But it's one of the joys of writing, and really, part of making a good character.