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The best way to avoid this problem is to understand your characters as deeply as possible, and remember that it is the character who should speak, and not you, the author. The things characters say...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/2714 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
The best way to avoid this problem is to understand your characters as deeply as possible, and remember that it is the character who should speak, and not you, the author. The things characters say and the way they say them are products of who they are, so knowing as much about them as possible will dictate their speech. This covers the usual gamut of physical, social, and psychological aspects that make people who they are. Are they educated, or school drop-outs? Is this language they're speaking their first language? What age group? Who are their friends? What is/was their family setting like? Any physical defects that could affect their speech? How do they behave in different social settings? When around strangers, they may guard their speech, but when around friends, they're free and open. What type of people do they like, or hate? Are they confident around men, and not women? Do they like to show off to be the centre of attention? Do they clam up to avoid attention? Are they thoughtful and slow to anger, choosing words carefully, or are they quick tempered? Rude? Polite? Indifferent? Friendly? Suspicious? Do they have particular phrases that they like? All of this (and a hell of a lot more) will change how and what is said by each character. Slang and accents are not needed (and can become very annoying) unless the character requires it. Delve into your characters, ask questions about them, understand them, and see the world through their eyes, not your own.