These rules must make some sort of sense and, most importantly, they must be immutable.
I bring this up thanks to an interesting article I read on io9 -- where the writer for a new film called 'Red Riding Hood' (the Amanda Seyfried one, not the Felicia Day, Syfy one) came up with an interesting take on why silver kills werewolves.
From the article:
Silver has a few good qualities. It conducts heat better than any other metal. It's incredibly ductile and malleable, which is one of the reasons why it's used for jewelry in the first place. Werewolves are equally malleable — they change shape easily. Although they're usually found in northern woods on cold, misty nights, they might have a problem with heat, but there has never been any mention of having to heat a silver bullet or knife before it's shot into a werewolf. What is it then, what makes silver so bad for werewolves?
There is one quality that silver has that isn't shared by many other decorative metals. It tarnishes. Silver has to be constantly polished or coated with something to protect it from the air. If it's left exposed, it develops a disgusting black crust that ruins the look of the silver. (Some people get silver especially for the disgusting black crust, but they have problems.) It turns out, though, that silver isn't reacting with the air. Silver is pretty nonreactive - staying the same in water, air, and most solvents. But tiny bits of an element suspended in the air combine with silver to make that blackened goop that coats it. What element is it? Sulfur.
Otherwise known as brimstone — that's right — the devil's element. Put together silver and sulfur and you get silver sulfide. Now, silver sulfide is not shown to be toxic to any other animals, but it's not soluble in water, and so can only be ingested. In a werewolf, it would lift from the silver and travel through the bloodstream of the animal, blocking off blood vessels and poisoning cells. And it is this that kills the werewolf.Now, I don't know if this flick is going to be any good (werewolf flicks as of late tend to be on the bottom rung of 'suck' these days... but that's just my humble opinion) but I love a good rule tweak like this. It's smart and it totally plays with the established canon (while still upholding it).
I doubt I'll get out to see Red Riding Hood while it's in theaters but I still wanted to give props where props are due.
The stronger your rules -- the more sense they make and the more consistent they are -- the greater the impact your world will have.
And if they happen to be really, really damned cool... even better.
P.S: What neat rules or rule twists have you come across in your TV/Film-viewing travels? Were there any that really made you sit up and go 'holy crap, I wish I'd thought of that!'?