Working out the rules for a supernatural threat

I'm busy developing a supernatural feature screenplay with the working title AKUMU. At the centre of my narrative is a supernatural threat. Now, I know why this force of antagonism does what it does and why it affects some people differently from others [there's a gender split, for reasons of backstory].

I've been working on AKUMU via a weekly feature film screenwriting class in Edinburgh, where ideas get workshopped and people ask all those awkward questions you push to the back of your mind when writing something. The logic flaws, the gaping plot holes - all stand exposed in their flawed gapiness.

One thing that's become apparent is I need to nail down the rules determining what my supernatural threat can and can't do. These rules have to be clear, easy to communicate and credible [within the context of the narrative, at least]. The need to clarity is obvious, ditto the ease of communication.

Credibility is wrapped up in issues of internal narrative logic. You can have the most outlandish stuff imaginable happening and your audience will buy into that - so long as it's credible in the world you've created. Break your own rules - or, worse still, have no rules - and you'll soon lose your audience.

That's why the Weeping Angels created by Steven Moffat for Doctor Who are so cool. Setting aside the technobabble about quantum locking and all of that, the rules for dealing with the Weeping Angels are simple: don't blink. If you're not looking directly at them, they can get you. Simple but brilliant.

There's a story I read long ago, probably apocryphal, that the  writer of Roger Rabbit spent years figuring out the rules for that stories collision of toons and real world characters. You can see a demonstration of how those rules work in this clip from the film version of Roger Rabbit.