You're right. There are several cameras all over the hall while the show is being shot. At any given point in time, the show's director is in a booth monitoring what's going on with each camera, and telling the camera operators where to point the lens, and how to frame the shots. Its actually pretty formulaic. The director and camera crew can expect to get shots of the audience as each contestant takes the stage.
I would imagine also that they would pick out people to focus on that are pretty, have been expressive, have something that will catch their eye. It would be second nature after a bit of practice wouldn't it, Milo?
I mean if one group or person is really getting excited or emotional about ONE act, wouldn't it make sense to focus on that group again in hopes of catching a usable moment?
I am much more open to the audience reaction being edited than Boyles performance being staged.
who knows WHEN that girl rolled her eyes? she may have done that about something her friend said before the show ever started and just edited.
the point I guess to me is that staged, edited, whatever; I enjoyed the performance, it touched me.