With the GM thing, you need not worry as we don't expect you to lead any missions by which I mean as a GM, not as a PC. Nova doesn't have a PC leader.

The only other aspect is that while in a mission, players can essentially spawn enemies and locations to an extent. In my mission for example, At no point did I introduce any gunships, but one of the other players decided it'd be fun to have a Blue Suns Mantis gunship on us... And he wrote one in. Even this is not expected, and largely reliant on the nature of the mission we are involved in at any given time. Some missions wont have nearly so major additions, some wont be suited to such things at all. It is an option, not an expected thing.