I try to write dialogue the same way that people talk naturally. If I’m writing a mechanic’s dialogue you’d better believe there are going to be F-bombs (disclaimer: I’m a mechanic by trade). If I’m writing a doctor’s dialogue I will try to class it up a bit.
I’ve actually used this as a plot device. Not a big part of the plot, but a part of it: Daecon, my main character in Finding Daecon’s Way, starts out as a rich, spoiled, arrogant brat with no couth whatsoever. As the story progresses (ie, as he finds his way) he becomes much more sophisticated and talks much more elegantly, though he does have his regressions occasionally. I would say that he’s only human, but really, he isn’t….