Good question, Ralph.  Now, let's see if I've got this right:  You've seen all these horror films, which lead you to believe or know that you can write a better screenplay than some of those that made it to the screen, which has lead you to decide to write a horror script, and you want to know how to write a good one.  The one thing that seems to be missing in the above equation is the part where you want to write a good horror script.  I'd make sure that that element is present because to write a decent screenplay -- of any genre -- you're going to need to put in some definite effort and time, and it's that little thing called "desire" (or "want" or "I gotta have it or I'll sleep on thumbtacks till I do").  But I digress.  (Hey, that's a great idea for a horror script!)

the Digression!!!!

(you'll never think of digressing again!)

Anybody reading this e-mail (and I know there are millions -- or hundreds -- or maybe twenty -- of you), please don't steal my horror script idea.  (Hey, I just got another idea for a horror script!)

SCripT Stealers!!

(you'll never steal a script again!)

Anyway, (sorry).  Anyway, let's get to the crux of your question, Ralph.  (Hey, that's another great idea for a script!)

CRUX!

(you'll never -- 

(Sorry, sorry).  Where was I?  Oh, yes.  Ralph, you want to know how to write a good horror script and sell it for a lot of money.  Well, the truth is you really don't have to write a good horror script to get everybody to come see it so you'll make tons of money.  All you really need is a good horror trailer (I'm not talking about the kind that go extremely slowly on the freeways, blocking and scaring anybody who comes up behind them -- which would definitely be a horror trailer.)  There are certain elements a good horror trailer needs:

ELEMENTS OF A GOOD HORROR TRAILER

1.  It must flash over and over again as if you were at a party with a strobe light and feeling nauseous after having a little too much of what everybody is having in the back.

2.  It must have a heart-pounding (possibly literally) and nails-on-chalkboard soundtrack (if that's what you want to call it) that shakes your internal organs to the point that they feel like they've turned to jelly.

3.  It must have an inordinate amount of screaming in it, mostly by vulnerable, extremely young and attractive and often not fully-dressed nubile, young women (don't worry if you don't know what "nubile" means; nobody does; they just use it with "young women" a lot).

4.  It must have a very, very fast pace with everything happening so quickly that it looks like the film has been sped up (which it often is), and so that lack of substance and quality can be shown with such speed that nobody notices that the film is bereft of any merit, whatsoever.

Once you have a trailer such as this in place, have no fears:  the young (not necessarily all nubile) will pour into the seats (being sure to tell their friends on their cell phones either to make sure to come and see the film or to stay very far away -- and often they do so during the film, God love them).

But if you want to go that extra mile, Ralph, and actually write a good horror script, here's a few suggestions that might help you along your good horror script writing way:

Suggestions for Writing a Good Horror Script

1.  Don't.

2.  But, if you decide you must...

3.  Find a way to make us really care about what happens to your     protagonist, who is fighting great odds (such as very upset and out of control mean people or less than people with sharp weapons such as their teeth).

4.  Don't worry about contrived twists.  The story is probably twisted enough.  But if you have a really good twist then, by all means, throw it into the mix.

5.  If your soon-to-not-be-alive sacrificial, supporting female is going to go into a dark cellar or attic or cave or anything else dark and not exactly safe, by herself, see if you can come up with at least one reason why she would do that.

6.  When in doubt, or you have no idea how to develop a good horror story with an enthralling storyline and authentic and engaging characters, just put in a lot of gore.

And don't forget those screams.

DcH