This is a question about dialogue and effective dialogue, can and should do more than just feature a conversation between two people (or the self - inner dialogue). It should add depth and personality, animate your characters, move the story forward, and feel as real as an actual conversation (even though you shouldn't write it that way.)
I feel it's always best to use a great existing examples so let's start with <PERSON> Of Mice and Men who created vibrant characters that are hard to forget. His characters come to life through dialogue and is a great study. We'll start with passages from Chapter 3:
...<PERSON> sat down on a box and <PERSON> took his place opposite.
"It wasn't nothing," said <PERSON>. "I would of had to drowned most of 'em anyways. No need to thank me about that."
<PERSON> said, "It wasn't much to you, maybe, but it was a hell of a lot to him. <PERSON>, I don't know how we're gonna get him to sleep in here. He'll want to sleep right out in the barn with 'em. We'll have trouble keepin' him from getting right in the box with them pups."
"It wasn't nothing," <PERSON> repeated. "Say, you sure was right about him. Maybe he ain't bright, but I never seen such a worker. He damn near killed his partner buckin' barley. There ain't nobody can keep up with him. <PERSON> awmighty, I never seen such a strong guy."
<PERSON> spoke proudly. "Jus' tell <PERSON> what to do an' he'll do it if it don't take no figuring. He can't think of nothing to do himself, but he sure can take orders."
There was a clang of horseshoe on iron stake outside and a little cheer of voices.
<PERSON> moved back slightly so the light was not on his face. "Funny how you an' him string along together." It was <PERSON>'s calm invitation to confidence.
"What's funny about it?" <PERSON> demanded defensively.
"Oh, I dunno. Hardly none of the guys ever travel together. 417
I hardly never seen two guys travel together. You know how the hands are, they just come in and get their bunk and work a month, and then they quit and go out alone. Never seem to give a damn about nobody. It jus' seems kinda funny a cuckoo like him and a smart little guy like you travelin' together."
"He ain't no cuckoo," said <PERSON>. "He's dumb as hell, but he ain't crazy. An' I ain't so bright neither, or I wouldn't be buckin' barley for my fifty and found. If I was bright, if I was even a little bit smart, I'd have my own little place, an' I'd be bringin' in my own crops, 'stead of doin' all the work and not getting what comes up outa the ground."
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CUT TO THE CHASE
Dialogue is very different than writing a conversation in that, you want to skip the pleasantries.
Notice, as soon as the men sat down, they started right into the conversation. <PERSON> could've easily started with something like:
"Whoo-wee! That sure was some work!"
"Sho nuff" said <PERSON>."
Instead he started as if they had already been in mid conversation:
"It wasn't nothing," said <PERSON>. "I would of had to drowned most of 'em anyways. No need to thank me about that."
<PERSON> said, "It wasn't much to you, maybe, but it was a hell of a lot to him. <PERSON>, I don't know how we're gonna get him to sleep in here. He'll want to sleep right out in the barn with 'em. We'll have trouble keepin' him from getting right in the box with them pups."
Even without knowing the story, you were able to figure that they had just come from doing some...not so nice task. We jump right into the action without having to suffer through any unnecessary banter that would slow down pace and extend the reading experience.
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ADD DEPTH & REVEAL CHARACTER
The best kinds of dialogue are the ones that help readers get a deeper understanding of a character. It's about the things they choose to say and don't say.
"It wasn't nothing," <PERSON> repeated. "Say, you sure was right about him. Maybe he ain't bright, but I never seen such a worker. He damn near killed his partner buckin' barley. 627