Dialogue, therefore, is not simply transcribed speech, but distilled speech. The “filler” we hear and overhear in live conversation (“phatic speech,” the opposite of “emphatic”) is edited away, even as the weight of implication is increased. It takes careful editing to create natural-sounding dialogue. Monologue in the right place can be a powerful tool, and some characters may be eloquent, but natural-sounding dialogue is probably ungrammatical; it may be full of slang, jargon, repetition, and various sorts of awkwardness. Generally, that means keeping things brief and paying attention to the rhythm of the sentences, which will itself reveal character, as will the failure or refusal to speak. A character who speaks several consecutive sentences might come across as a windbag (which of course may be your intention) or the author's puppet. Often the first and last words, phrases, or sentences of a character's speech can be eliminated. The character's intentions and preoccupations will shine through because of the leaps they make in their dialogue, as well as the the things they can't bring themselves to say aloud. Even rote exchanges, however, can call up images. A character who says, “It is indeed a pleasure to meet you” carries his back at a different angle, dresses differently, from a character who says, “Hey, man, what's up?”

Janet Burroway, Writing Fiction, Tenth Edit…, loc. 172