Having flown quite a few times as the "shotgun", it very much depended on the Training Captain. In some cases you were invited to help the new boy with walk rounds, paper work, monitoring etc. However one trainer (and he was the Chief trainer) informed me that I was in effect a spare tyre, and had no function unless he was taken ill, which case I was to assume command. If there was a spare seat down the back I was to sit in it. We're back to talking egos, aren't we?