Actually the way it would work would be that they would both inherit from a common class (prototype critter?) and therefore would be called the same way, via say the hypothetical "move" method. But, you would override the move method to make it function differently depending on the type of NPC or to add an interface for the PC's. But you would still call it by the same method name. i.e. move

Wait! This isn't a Parachute, this is a Backpack!