We often write dialogue simply to move a story forward.
(Note: I'm not just talking about fiction here. Nonfiction is chock full of story, as is poetry, music, and nearly everything else we write.)
There's nothing wrong with that, either. Most sentences are written for the sheer purpose of persuading readers to keep reading. Sure, each sentence contributes to the whole, just like each passing minute is a crucial component of a day. But just like you won't remember every little thing you did or thought today, not all of your sentences should be showstoppers.
Which is why it's so, so important to make sure your dialogue is as embodied as possible. If you're not sure what I mean by that, read this excerpt from The Wind in the Willows by Kenneth Grahame:
"But what I wanted to ask you was, won't you take me to call on Mr. Toad? I've heard so much about him, and I do so want to make his acquaintance." [spoken by Mole]
"Why, certainly," said the good-natured Rat, jumping to his feet and dismissing poetry from his mind for the day. "Get the boat out, and we'll paddle up there at once. It's never the wrong time to call on Toad. Early or late, he's always the same fellow. Always good-tempered, always glad to see, always sorry when you go!"
"He must be a very nice animal," observed the Mole, as he got into the boat and took the sculls, while the Rat settled himself comfortably in the stern.
To our modern, post-Hemingway ears, Grahame's writing is a little on the wordy side. Both Mole and Rat say more than is absolutely necessary. (This would be even clearer if I were to a quote a longer passage, but I want you to keep reading, so ...)
And yet, even my eight-year-old son is able to stay on track and emotionally invested in this story. Why is that?
I think the answer is found in the dialogue: specifically, the way that Grahame embodies his characters' speeches. Notice how they rarely just talk. Or, to be more accurate, we rarely see them just talk. Grahame helpfully breaks up their dialogue with descriptions of what the characters are doing:
... said, the good-natured Rat, jumping to his feet and dismissing poetry from his mind for the day.
... observed the Mole, as he got into the boat and took the sculls, while the Rat settled himself comfortably in the stern.
I suggest to you that these casual phrases – unmemorable phrases, I might add – are precisely what keeps our interest. As informative as the dialogue is, it neither sticks nor catches our attention unless it's situated in a physical, action-filled setting that we can see.
This lesson can be counterintuitive. A lot of us instinctively believe in the primacy of the word. We're convinced that what really matters is what is said. But that's only half the story, isn't it? (And maybe not even half.)
So, next time you're writing, throw in some embodied dialogue. It's a surefire way to catch and keep your reader's attention.