Where to begin?

I'm trying to figure out how the script should begin. In many ways, I think it would be better to begin by introducing the setting and the characters, and then gradually bringing in the horror elements. However, the old slasher movie convention of opening with a shocking scene might serve as a way of buying time. The original Texas Chainsaw Massacre, for example, begins with such a creepy intro that it makes the rest of the first act seem a lot more tense and unnerving than it really is.

On the other hand, it might be better to start out the script relatively calmly and slowly draw the horror out of the context. Most of my ideas for an opening set piece are somewhat unrelated to the story. What I think I will do is write a shocking opening scene and see how it works when the whole thing is completed. I have some ideas for that.