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The hardest thing for most writers to get right is natural dialog. Characters often sound stilted and unreal. Dialog should seem real to the reader, but should avoid the umms, errs, and false starts of real speech.
Go out into a public place and eavesdrop on conversation, record two people having a conversation, or try reading a verbatim transcript of a live show. You will find that the it is almost unintelligible. You don't want your dialogue to be this true to life unless you are trying to convey a quirk about the character, such as a speech impediment, low-level education, or some psychological problem.