It can be tempting when writing realist dialogue to carry out the assumption that when two people are speaking, they are speaking to each other. Like checkered tiles in a kitchen, this kind of dialogue is a pattern that the reader has seen before, many times: a pattern they can recognize without looking closely, and so barely see. When I first started writing fiction, I felt that this was the only correct way, this direct call-and-response where each hears the other and then in turn is heard. Because it does seem, most of the time, that this is what takes place.
But in reality, nobody ever talks to anyone else. What speech actually achieves is a communication between one person and that person's idea of the other. Most of the time there is no difference, no discernible difference, between such verisimilitude and the truth. But the best dialogue will manifest this disparity in subtle, slender ways. It will show how, in speaking, we fail to speak.
Take this exchange at the end of Arthur Miller's Death of a Salesman, between brothers Biff and Happy Loman, in the wake of their father's death:
the man didn't know who he was.
HAPPY (infuriated): Don't say that!
BIFF: Why don't you come with me, Happy?
HAPPY: I'm not licked that easily. I'm staying right in this city, and I'm gonna beat this racket! (He looks at Biff, his chin set.) The Loman Brothers!
BIFF: I know who I am, kid.
Each son's ideas of their father have contaminated their ideas of one another. Each in their own way asks the other to join them, and yet neither actually hears the request. Biff only hears that Happy is blinded by ambition in the same way that their father was, while Happy only hears that Biff is his father's very definition of a quitter. It's a pattern that the reader doesn't see often, that moves them to lean in and look close.
So remember when writing realist dialogue that while fiction can pierce through the surface of another's mind, conversation only ever delivers glancing blows. Because one can never really know, through speech, how the other thinks. There's just not the time, the space, nor the solitude to forget yourself in conversation with another person like there is with a good, long piece of prose. An elegant speech, a heartfelt confession or a passionate fight—they all might dent the surface, might even leave a permanent mark, but they won't break through. They just won't. That's why we need fiction, and why good dialogue is so rewarding: it makes us crave exactly what we have before us, like hunger at a table filled with food.