Below is the famous quote set into the beginning of most editions of Mark Twain’s Huckleberry Finn on the different dialects used in the novel:
IN this book a number of dialects are used, to wit: the Missouri negro dialect; the extremest form of the backwoods Southwestern dialect; the ordinary “Pike County” dialect; and four modified varieties of this last. The shadings have not been done in a haphazard fashion, or by guesswork; but painstakingly, and with the trustworthy guidance and support of personal familiarity with these several forms of speech. I make this explanation for the reason that without it many readers would suppose that all these characters were trying to talk alike and not succeeding.
This is quoted from this free online full-text of Huckleberry Finn.
This is a valuable clue into how to prepare for either your audition or your recording session with p4–not all characters can (in fact, should) sound alike.