Although profanity has become a lot more widespread in mainstream theater, there are still those who feel uncomfortable hearing "bad" words used on stage. I can understand this to a degree; sometimes writers use strong language solely for its shock value and not to actually say anything or use it too often and it loses its potency. But there are some playwrights, like Tennessee Williams, who successfully utilize profanity to make a social statement or define a character.
The more realistic, the more unnerving. How can you watch someone get beaten to a pulp in front of you and do nothing? How can you watch a man hit his wife and sit silently by? You can when watching a production of it, but humans empathize so deeply with physical pain that we often have a physical reaction. It's similar to seeing or thinking about someone's fingernail getting bent up—really makes you physically cringe, doesn't it? Seeing an actor getting hurt onstage, hearing them calling out in pain, and not being able to take action to make it stop because it's fictional really gets to some people.
Even in our more modern society, we're been programmed to think of the naked body as something dirty, or at the very least, private. In my life, nudity is an intimate and personal thing, and seeing someone up there who (as long as they're not breaking the 4th wall) is apparently oblivious to you makes you feel guilty. Seeing someone completely exposed and vulnerable simultaneously fascinates us and makes us embarrassed on their behalf.
It sure does sell! You can tell by looking at Spring Awakening. Although you can attribute much of its success to its alternative/pop/rock score, what made it the topic of everyone's gossip is the fact that two characters engage in a very graphic portrayal of sex twice in the course of the show. I remember seeing a production at the Ahmanson here in Los Angeles, when I was a junior in high school. I had heard things about "the scene," but didn't know what to expect. There was tension while I watched, hyperawareness that I was surrounded by hundreds of people, all watching two people have sex right in front of us. At intermission, there were many stilted and awkward conversations, usually beginning with, "Soooooo…" Of course this is a version of events told by an already awkward, hormone-ridden teenager. Still, people watching that degree of intimacy usually reserved for behind closed doors creates awkwardness and a feeling of shame.
Yep, you read that right; silence is one of the most profound ways to unnerve an audience and make them shift uncomfortably in their seats. I'm not talking about "Oh crap, I forgot my line…" silence (although that can be awkward too). I'm talking about the Samuel Beckett-style silence that makes the audience hyper-aware that they are sitting in a theater, surrounded by other humans, watching a play.
Essentially, Beckett aims to show us the absurdity of human existence in our desperate search for meaning and our individual isolation. Bleak, isn't it? I recently had the fortune to see John Hurt in Krapp's Last Tape, and was deeply struck when the lights slowly faded up to reveal Hurt sitting behind a desk, unmoving…and remaining unmoving for about 5 minutes.
Within that time, you could see people moving about in their seats, leaning over to whisper to each other, and looking around at others in a flustered manner. Audiences usually attend plays for entertainment purposes, which is why they become so uneasy when things don't go as expected. Having their idiosyncrasies pointed out in front of the rest of the audience removes them from the escapism for which many of them were in attendance.)
If used incorrectly, namely just to shock or appall, audiences will turn away in disgust. But making people uncomfortable isn't always a bad thing. When used for the purpose of enhancing the story or characters, some of these things really do make a bold statement and deeply affect the audience.