Sometimes my characters cuss and generally act in a disreputable manner.
Actually, I try not to use epithets or cursing when I write. This is partly stylistic, but mostly this is because I feel that the curse word has lost its power in the world. In the microcosm of the novel, I feel that is certainly true, as evidenced by the fact that once I'm a few chapters into an Elmore Leonard novel I stop noticing the words and start really grooving to what he's talking about. This is cool because it means his writing is working, but it also means that a character saying shit no longer has an impact. It's just how (many if not most of) his characters speak, how they express every thought, not just the extraordinarily passionate ones.
No one uses vernacular cursing better than Elmore Leonard. It lends authenticity to his streetwise characters. It's part of his 'voice', that ineffable part of us that differentiates writers and allows us to tell stories in our own way. It's part of why two writers could write the same book and you might never realize it because as long as those two writers know their voice they won't tell the same story the same way.
It's not right or wrong, it's just an observation. In both writing and life, I come down on the side of the equation that says "do more with less". I don't eschew cursing entirely, but I want it to be an expression of passionate feeling. The fewer times a word appears, the greater its impact. And I want any word I use to be evocative of the feeling it's expressing. "Oh crap" takes you immediately back to a time when dropping the top scoop from your ice cream cone is the worst event you can imagine. "Oh shit" doesn't do that -- at least not for me. It doesn't have the power of it's adolescent substitute.
There's a famous comedy routine by the late George Carlin that discusses the seven words you cannot say on television. The most famous portion of the routine is where he rattles off all seven in rapidfire succession, but I find that in the focus on the profanity, his thesis gets lost. Here's the part of the lead-in that gets lost in the discussion...
"I love words. I thank you for hearing my words, I want to tell you something about words that I think is important. They're my work, they're my play, they're my passion. Words are all we have, really. We have thoughts but thoughts are fluid. Then we assign a word to a thought and we're stuck with that word for that thought, so be careful with words."As with anything, there's a right way and a wrong way to curse in fiction. The right way is epitomized by Elmore Leonard. The heavy use of slang and earthy expression adds depth, pathos, and reality. Robert Parker's Spenser novels are another good example of how to do it right. The wrong way to do it... well, I won't single anyone out, but like Potter Stewart said about pornography it's difficult to define but you'll know it when you see it.
-George Carlin "Class Clown EP"
My feelings about profanity are mixed. All too often, it's not a Leonard or Parker quality of characterization but an overused device that thinly veils bad writing just as all too often it's a beard for unfunny comedy. Shock value will only get you so far before someone wants you to back it up with content. In this as all things, content and context share the throne.
Carlin said "Be careful with words". Every writer should write that on the wall above their writing space. In real life and fictional, words have power and should be used with caution. Used properly, the right words - even curse words - can help your story, but the wrong ones at the wrong time will kill it.