Instead, they drive their half of the story by taking the characters that they have, characters which have been approved by the individual in charge, and reacting to what the world throws at them. They do not have any control outside of their individual characters, but how they use those characters is as impactful, if not more so, to the party than what the world itself does.
The problem is, not all characters fit a setting, and if they don't it ends up being world-breaking. I'll use a D&D example here, because it's the best way I have to describe it. Say there's a party of 4 people: all of them are level 3, and all are basic, core classes. At this point, a CR (challenge rating) of 3 would mean that any one of the party members should be able to solo it, albeit with a 50% chance of losing. That means, with all of them there, they should be able to take something like that without a problem. Now, if the enemy is CR 10, a level 3 party should get crushed by it. That's something like a massive boss fight that isn't supposed to be tackled yet, or something that they need to use some other method to get past.
Now, imagine if the level 3 party suddenly gets a level 11 ally. Everything that they might have had to think about or use some kind of tactical thinking to get past is now void as their level 11 ally blows up everything in their way. It takes any concept of story and balance and throws it out the window, forcing the person in charge to compensate for it. The problem is, you can't effectively do so, since the ally is too strong for anything to be thrown their way that won't immediately kill the party.
Conversely, should the level 3 party suddenly have that same ally appear in front of them, but this time as an enemy that they cannot ignore, there's nothing that they can do. They will be stuck in a situation where they can either ally with the much stronger individual or perish, because there is no middle ground when you are so vastly overpowered.
Done in by the dubious doings of destiny.
20-Jan-2017 16:28:29