There was a time, not that long ago, when certain Anglo Saxon words simply did not appear in print. They didn’t appear in conversation either, unless the speaker didn’t know better—a kind of social misfit.
Norman Mailer’s classic WWII novel, THE NAKED AND THE DEAD, posed a serious problem to his publishers. Mailer’s characters were combat soldiers, a mixed bag of men with nothing in common but fit bodies and compulsory military service. Mailer captured their voices brilliantly; we heard them brag, complain, grunt. They talked the language of men among men. But how was a publisher to present this talk to a reading public used to more polite exchanges?
(Mailer, whose book would be acclaimed as the definitive WWII novel, was accused by his envious peers as the author who couldn’t spell “fuck.”)
We’re obviously more permissive now and once-taboo words have lost their shock value. So new authors may spice their dialogue as tools of emphasis or character tics. But treat these as seasoning to the main dish. Used judiciously in dialogue, crude or salty words can further define a personality or heighten a dramatic situation. Used indiscriminately too often or inappropriately, they can detract from your story and reflect on the author’s own poverty of language.
There is one absolute about this kind of language. It is never to be used in the narrator’s voice. In fiction, it must always be neutral. Internal dialogue, i.e., a protagonist’s thoughts, may be salted with an original expletive if the character has been already established as a somewhat unorthodox hero or heroine.
In serious non-fiction, the old rules still apply. No naughty words unless you’re writing an etymological history of improper language, and I’m pretty sure that’s already been done several times.