Why is dialogue so important, you may find yourself asking. Well, that question almost answers itself. In order for anyone to find anything out, questions must be asked an answered- that's a form of dialogue. Basically, dialogue can extend necessary exposition to the reader and help define the characteristics of the speaker.
Good dialogue will give the reader an insight into the character speaking, as well as the listener. Just as we can get at least a small idea of who a person is from the way they speak to us in real life, it happens the same way on the page.
famous short story by Ernest Hemingway"Hills Like White Elephants"
Here's an excerpt:
‘It’s really an awfully simple operation, Jig,’ the man said. ‘It’s not really an operation at all.’
The girl looked at the ground the table legs rested on.
‘I know you wouldn’t mind it, Jig. It’s really not anything. It’s just to let the air in.’
The girl did not say anything.
‘I’ll go with you and I’ll stay with you all the time. They just let the air in and then it’s all perfectly natural.’
‘Then what will we do afterwards?’
‘We’ll be fine afterwards. Just like we were before.’
‘What makes you think so?’
'That’s the only thing that bothers us. It’s the only thing that’s made us unhappy.’
I don't want to spoil the story, but you can see a couple of things going on here in this piece of dialogue. We know that the woman is due for an operation, and that the man seems more confident in it than she is. You hear her doubt, her worry, and his insistence. This exemplifies the narrative power of dialogue.