Well, obviously we could decline to participate, and as the play went on more and more people obviously did so. And to respond to a previous post, of course there were no obvious consequences in not participating. But there were consequences, to the extent that one tended to feel a bit guilty for not joining in, especially when entreated to "do better", or yell louder.
And in my view, the audience in a theater can't be compared to a TV audience in the sense that the TV audience is not influenced or affected by the response of strangers sitting around them. I vividly remember the scene in the film where people opened up their apartment windows and began to scream the famous scream. It was great. This didn't feel anything like that.