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People should talk as people talk. If the flow is in character with, well, your character, then it's ok, it won't be less credible. But if, say, a street though is talking like a Duke... well, that...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/2580 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
People should talk as people talk. If the flow is in character with, well, your character, then it's ok, it won't be less credible. But if, say, a street though is talking like a Duke... well, that'd not happen in real life, would it? It can be _always perfect_ for a character that is that kind of person; it would add to the story, let people know about the characters. It can be occasionally perfect for someone who has the right level of education, the right personality. Could be mock perfection, or ironic perfection.