M. Night Shyamalan has been a wildly successful writer in Hollywood, and it's almost all due to his ability with plot. Shyamalan is a master of the reveal, and in the blockbuster world of mainstream movies, that is the best talent to have.
Signs has nowhere near the quality of The Sixth Sense. But it does show some of the writer's techniques.
Plot comes from hidden opposition. One reason the plot in Signs is not as strong as the one in The Sixth Sense is that Shyamalan exposes the opposition fairly early. Partly that is because Signs is a horror film, a form that is based on the attack of the opponent. When a relentlessly attacking opponent drives your plot, you have a lot of problems hiding the opponent's power and setting up reveals.
Shyamalan tries to delay showing us the opponent as long as possible by tracking a series of signs that a possible opponent is attacking. But that gets old pretty fast.
To make up for a genre that tends to kill plot (along with everything else in the story), the writer sets up a number of traits and facts about the characters that will pay off in the plot at the end of the film. These traits and facts - like the son's asthma and the brother's ability as a home run hitter - are improbable enough that you can tell when you first see them they are set ups. You know they will be paid off, you just don't know how. Still, when they finally are paid off, it is pleasing to the audience. And Shyamalan's greatest strength as a writer is his ability to hold out as many plot payoffs as possible until the end of the story.
The other major technique that Shyamalan uses is connecting supernatural stories with realistic psychological weaknesses in his characters. This technique, pioneered by Stephen King, is difficult to do well because it is inherently ridiculous. Typically, a character that has undergone severe trauma in his personal life must then confront a number of sensational, otherworldly events.
In Signs, Shyamalan doesn't completely overcome the silliness of the technique. His hero, after all, is an ex-minister who finds his faith by defeating extra-terrestrials. True, I know a couple of ex-ministers who had this experience, but it is rare.
Shyamalan's main tool for taking the stink off is humor. By making fun of a lot of the conventions of the genre - like wearing aluminum foil caps so the aliens won't know what they're thinking - the writer makes it ok to accept the fundamental premise that aliens are attacking. He also alternates serious or scary moments with funny ones, a difficult flip in tone that, when done successfully, makes the characters seem more real.
What is most important to understand about this technique is that you can't be afraid to use it. Yes, it can seem stupid and contrived. In Jurassic Park, it made no sense that the lead character hated kids. Except that it gave him a character weakness he had to overcome by working through the plot. Audiences don't just want to see a plot played out, no matter how ingenious. They want to see a character deal with a personal weakness and come out better on the other side. Even a contrived character need is better than none at all.
