Couple of things wrong here:
1. You're not telling a story, you're "telling a scene".
2. You're not telling your own story.
There is no beginning, middle and end to your script, so you're telling a scene. We don't know who the giant is, what he's done or what he might do. You've given no real detail as to whether this is a friendly giant or a terrifying giant. There's no sense of time or place; sure, a giant in a cave generally belongs in some sort of fairytale land - but David and Goliath is grounded in a "real" Biblical world.
You're relying too heavily on a reader knowing the story of David and Goliath to fill in all the details surrounding the script's too characters, the world it takes place in and...well, pretty much everything else. If you're assuming that "everyone knows the story" - then what's the point of this script?
The only reason for a reader familiar with a tale to take a look at the umpteenth adaptation of that story is to see or read something new. Put a twist on it, develop new and interesting motivations for David and Goliath - do something or attempt something innovative! Otherwise, again, what is the point?
On the other hand, if the reader has never encountered the story of David and Goliath before...well, how lost would he be when you provide...nothing really. Here's some giant resting in a cave and some little snot-nosed punk shows up out of nowhere to bother it - and then the script ends. And what is the point?
Any story, long or short, script or prose of screenplay, needs a beginning, middle and end or it's just a scene. Sometimes that works, sure, but generally readers want to invest in a full story.
Good luck, mate.
