When creative writing students turn in their first short story or dialogue exercise, the teacher’s response is frequently, “All of your characters sound like you.”
The writer had certain points to communicate via dialogue and distributed them amongst the characters as though their manner of speaking is interchangeable. Or, as the teacher might say, “You should be able to tell which character is talking by what they say and how they say it.”
Several student responses are likely: (1) A dozen synonyms for said. (Yes, there’s a difference between “he said,” “he yelled,” and “he whispered.”) But they don’t help if the words that are said don’t sound any different in tone, structure, word choice, accent, and focus than the three other people in the conversation. (2) The student thinks up a list of eccentric phrases and distributes these amongst the characters, rather like dealing out cards, so that EVERYONE TALKS FUNNY. The teacher is likely to say, “The people sound like they just escaped from a carnival freakshow.”
One of the hardest things for a writer to do is getting to know his/her characters so well that the way they talk arises naturally out of the person. People talk differently because they are different. The writer’s at a disadvantage here if s/he hasn’t spent any time listening to how “real people” express themselves. Some use slang, some have accents, others speak in short sentences while a few speak in paragraphs. Children sound like children and are influenced by fad words from school or (in modern times) words from texting. Older people may use terms from 40-50 years ago that young people may never have heard, as in “You ain’t got no gumption.”
One way to figure all this out is by reading the works of authors who write great dialogue. TV viewers and critics used to say “‘The West Wing’ has great dialogue.” Listen to a few of these shows and figure out what Aaron Sorkin did to make his characters’ dialogue memorable. Here again, the characters all had their issues, likes and dislikes, fears, joys, etc., so what they said fit who they were.
Resist the urge to pepper conversations with small talk. That slows down the story even if it does sound just like a conversation you heard in a store or on the subway. You are advancing the plot, not shooting the breeze. Read your words aloud. So they sound like they’re words to be read or words to be spoken?
If you look up “writing dialogue” online, you’ll find some decent advice that’s almost as good a learning by reading well-written novels.