Q: What, specifically, are you looking for on that first page that will keep you reading on?

The ironic thing about screenplays is that they are designed to be movies, and of course when they are movies, hooking the audience with the first few minutes won't be of key importance; you've gotten their money, they are a captive audience, and the opening can be slow as hell if the movie as a whole works.

"Casablanca" opens with a bunch of stuff on the screen to read, big clunky exposition setting up its story. On the page, it probably wasn't much of a gripper. On the screen, it all works.

As a reader, I'm a captive audience too, because I have to read every page of every script I get, that's my job. But obviously with a lot of people in the business hooking them is going to be a major concern; you want an exec or an agent to pick up your script and get so into it that they are cancelling meetings just to be able to finish it in one sitting.

Of course, there's no easy answer to how to write a perfect opening. But in general, you need to give a real sense that you are about to tell an interesting story. Jump right into an intriguing situation. Introduce a fresh, dramatic, or funny main character. Though it's somewhat overused, opening the tale with a chunk of action from later in the movie, and then jumping back three days (or whatever) can grab a reader, because it gives the subsequent scenes, setting up your main character, a real sense of where the script is going.

Clunkier, common things to probably avoid (unless you can pull it off really really well) : Dream sequences. Bits in which you pan across the main character's messy room, and then stop on him waking up. A lot of awkward voiceover, overexplaining the backstory.

Here's an exercise. Flip around cable channels watching random movies/TV shows. What happens that makes you want to watch a little longer? What are the things that hook you?
(gators on a helicopter)