To state the patently obvious, the opening of your screenplay is of vital importance. Page one, scene one is when the reader begins to form their impression of your story. It's a tremendous opportunity to hook them in for the next 90-to-120 pages, and if you fail to get them then the odds are you won't catch them later and they'll stop reading.
You have to achieve so many things in that first scene. Tone. Style. Genre. Premise. Narrative World. As soon as you think that you have the perfect opening...the opening the story demands, the opening that is exactly right...do another version. Take it apart. Throw it out. Try something else. Go back to it. See what sticks. Perfect it. It's like giving birth. There's a reason parents try to bring their children into the world as peacefully and supportively as possible. After all, there are plenty of opportunities to scar and corrupt your child later. It's the same thing with your story.
Toward that end, here are five random things to keep in mind when crafting your opening.
1. Introduce your protagonist as soon as possible. If you're not bringing them in until page 8 and/or introducing them after a plethora of other characters, that's too late. Readers and audiences inherently enter your story with uncertainty because they don't know what to expect. They immediately start looking for the character to focus on and orient themselves to; they're begging to meet your protagonist so that they feel safe.
Think of your lead as a tour guide. If people sign up for a tour and meet at the desginated start point to find no one there or a group of people who are indistinguishable, confusion and chaos will soon ensue. Readers and audience members want to be lead, so lead them. They're going to want to latch onto the first character they see...any port in the storm...and if your protagonist doesn't come in until after a bunch of people or other plot points are introduced, they're going to be wasting time investing interest in the wrong person and will then feel jarred and probably unsatisfied when they realize the story isn't about who they thought it was about. Also, on a practical level, the sooner you introduce your lead, the more time you have to build an emotional connection with the reader.
2. Make sure to communicate the genre. Convince the reader that you will deliver on the core requirements of that genre. If your script is a comedy, that opening scene better generate laughs or you're dead in the water. If it's horror, dread and menace better start seeping in.
3. Create conflict immediately. Can't stress how important this one is. Conflict is the building block of every scene, regardless of genre. If there is absolutely no conflict in a scene, it shouldn't be in the script. Lack of conflict bores people, and the very last thing you want to do in your opening is bore people.
4. Don't start with a flashback. This one drives me nuts. You can't flash back to something if you haven't established a present timeline and narrative world first. When I read a script that reads...
...it makes me want to set the script...and myself...on fire.
5. Make sure the end of your first scene contrasts in some way with the beginning of your second scene. It doesn't matter what the contrast is per se, just make sure that you have one. If your first scene is set in a hotel room, don't begin your next scene in a hotel room. Provide a visual contrast. If you open with an explosive action sequence, make your second scene a quiet character moment. Just give audiences some kind of contrast to create kinetic energy so that fatigue doesn't set in.
Most importantly, take time to get your opening right. There are bad scripts with great openings, but there are no great scripts with lousy openings.