Sal just started doing "post produced" work, meaning he wasn't doing the camera live and recording while watching a demo. It's possible, and probably easiest if you're just starting, to do your own camera beforehand and then record.

The advantages would be knowing how the action is going to happen, so you wouldn't be so lost for a first time caster. I don't know if you could sound scripted if you know too much about the details of the game thoguh

I'm not a caster, and I would definitely try to talk to someone who has a lot of experience casting (like Sal etc as VoodooChow suggested).