Back in the late 70s, Liza Minelli was in SF for out of town tryouts for something that was *then* known as "The Act". I dont remember what the title was when it finally made it to NY, but it was about a cabaret performer looking back on her life. It was interminable. Three hours after the thing had started, we were *finally* at the end of "the act", and Liza, in character, is taking a bow. We knew it wasnt the end, because there's one more song listed in the program. She then comes down to the edge of the stage and sits, very Garland, and says "You've been such a lovely audience. You know what I feel like doing?"
Someone in the upper balcony yelled out, 'I dont know, but I'd like to get home sometime tonight!" The theatre fell apart laughing. Liza laughed with them, then enjoined, "Wait ten minutes. We can share a cab."
Now thats class.
But as Margo points out, booing is much stronger a tradition in Europe than here. Audiences there are less forgiving, but they also boo at the appropriate target. There was a horrendous FLEDERMAUS in Germany three or four years ago, wherein the director publicly stated that he didnt think it was a very funny opera and proceeded to stage the third act in a concentration camp -- and while the audience politely applauded the singers, they were merciless with the director/designer team, so much so that the two had to leave under police protection by a side door. (And yes, it was that bad, folks -- its out on DVD, if anyone's interested).
Bottom line? If something's bad, let them know. But make sure you're letting the *right* people know.
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