I would definitely be fired in those instances. Maybe not grease or Mamma Mia book 2, but certainly most of the books I've played no way I could have gotten through them cold without some work at home. But like I said above some of the cats I've seen who do this for a living can do that. It's impressive really. Regarding positions, I make a lot of pencil marks as well. It's probably obvious for everyone who does this regularly but I generally look ahead a couple of bars to the highest note and use that to pick a position, coupled with the key signature.