When attending live theater, it's a good idea to arrive at least 30 minutes before curtain. Many theaters won't allow you to enter once the curtain rises, and others will happily give your seat away if you're not there immediately prior to the show.
Always eat before you get to the theater. You can usually buy chips, candy, and other snacks in the theater lobby, but the rustling of the packaging won't endear you to other theater goers, whether you're attending a live performance or cinema.
Don't leave your cell phone on during a live performance (or at the cinema, for that matter), and never, ever actually answer a call when you're in the audience. Although it's unfortunately too common today, this is the utmost in rudeness, especially in live theater.
