If a northerner attempts to imitate a southern accent it will be taken as an insult. The accent comes along naturally and without conscious effort. Then it becomes just fine. The key thing is be yourself, unless your a nasty person, then don't say a word and don't even go out into the public.
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Southerners are thought of as slow moving people, which many are, but they are very quick witted and can laugh at themselves as long at they are the ones joking about it. Outsiders really need to watch their manners especially around women. And don't chuckle when you hear a middle aged man refer to his father as Daddy or his mother as Momma. Somethings just weren't meant to be funny.