Its funny you mention the audiences reactions. Ive been to a few tapings of AGT and they pretape ALL reactions before the show!!!! A producer comes out, and will say, "Pretend you just saw something so funny!!" or "Now, act shocked, then laugh, clap and stand up!" And there are cameras swirling all over the audience filming these reactions that they'll splice in later. You're doing this while looking at an empty stage!! Its all manufactured, but actually fun.