I'll add that my personal preferences as a GM (in all the games I run) come into this.
The two points are:
1) If it's not impossible for a PC to do something, then he can try to do it. The chances may be incredibly low, but the chance is there.
2) Friendly NPCs don't get to roll to do something a PC (or more than one) have rolled and failed. NPCs shall not upstage PCs. If the PCs are expecting the NPC to do something (such as in a combat with a lot of soldiers, or he's an expert with a skill no PC possesses) then the NPC rolls normally.
So, if anyone in the HUMVEE is an NPC, I don't roll for for him until all the PCs who could roll have done so and failed.
|