Introduce the Subject Early

Subject (def.): a two or three word description of what your story is about.

It's important to introduce the subject early. This helps to orient the audience. In "500 Ways to Beat the Hollywood Script Reader," Tip #228 is, "Foreshadow where your story is going in the opening sequence."

Some examples (mine):
"Thelma and Louise" is basically a road trip story and the opening scene is a winding road leading off into the distance.

The subject of "Jurassic Park" is scary dinosaurs. An early script began the story with the recruitment of Dr. Alan Grant. BUT, when the film hit the theaters, the opening scene had changed to a dinosaur breaking free from it's crate and gobbling down one of the workers, hardhat and all.

The subject of "Casablanca" is escaping from Casablanca. In the opening, the narrator says, "...[in Casablanca] the fortunate ones, through money, or influence, or luck, might obtain exit visas and scurry to Lisbon, and from Lisbon to the New World. But the others wait in Casablanca -- and wait -- and wait -- and wait." A page later, the pickpocket says, "...along with these unhappy refugees the scum of Europe has gravitated to Casablanca. Some of them have been waiting years for a visa." A page later, refugees gaze forlornly at the plane to Lisbon and young refugee Annina says, "Perhaps tomorrow we'll be on that plane." So, the Epstein brothers not only introduced the subject early, they hammered it in.

Exception to the rule:
In the Sixth Sense, the subject is seeing dead people. This isn't clearly introduced until page 53 when Cole says, "I see dead people." But Night strongly foreshadows it several times:
On page 15, Malcolm discovers that Cole is saying the mass for the dead.
On page 24, Cole give realistic biographies for two of his "dead" toy soldiers.
On page 28, Cole knows that Mrs. Marschal is dead.

So, while the rule of thumb is to introduce the subject early, you can finesse. Especially if you can write as well as M. Night Shyamalan.