We used to have people singing along to the songs in FOREVER PLAID all the time. It is strange when you're not used to it.
It's really quite simple... if you're disturbing others, cut it out. There are shows at which you should expect some audience involvement (any of the Disney musicals, fr'instance), but if someone's
getting out of their seat, well, you've crossed the line.
As a somewhat over-enthusiastic theatergoer, my past sins I confess today:
1) as a young lad, I caught the revivial of
Joseph (starring Michael Damian and Kelli Rabke) at the Minskoff. This was the first time I saw a show where I knew the entire score by heart, and was mouthing along to myself (or so I thought). At curtain call, the lady "of a certain age" sitting next to me turned and asked "so, when are you joining the cast?" I like to believe that I was not disturbing her too much, and that she was more amused than annoyed.
2) in later years, at the umpteenth
Rent performance, I would just turn to the people behind me and say "smack me upside the head if I'm bothering you. You paid to see the show on stage, not my re-enactment."
3) As a young broke college student, I would frequently catch shows from standing room. This was the ideal location for an over-exuberant audience member, since I wasn't in front of anybody, and if I took a step back, nobody could hear me either. The ushers and bar staff of the Palace (when
Beauty and the Beast was playing there) and the New Amsterdam unquestionable have, er,
interesting memories of the goofy-looking Jewish kid who was acting along to the whole show.
To defend myself, I tried to be aware of the people around me, and the late Clark Gesner, after the last performance of the revivial of
You're a Good Man, Charlie Brown, actually told me how happy he was to see someone so thoroughly enjoying himself at his show.