Why would you not say "shit" during an interview? I wouldn't make it my every other word, but I find nothing wrong in talking to someone as I'd talk to them on a daily basis if I were hired.
I have no idea what the interviewer's attitude toward casual swearing is. Once I'm hired, I'll cuss like a sailor, cause by then it's really too late for them to do anything but ask me to keep the profanity down.
Because, and maybe I'm old-fashioned, I don't think it is the appropriate place for it. Besides, I try to find other ways to express myself. Swearing, to me, is lazy and I don't want that to be something someone remembers me by.
eh, i think the stigma associated with swearing has started to wane over the last decade or two. Admittedly if you swear every other word, you're not a very good communicator, but the occasional, impassioned (and well selected) swear word can be endearing, even in a professional environment.
