Don't just have a character explain the story or say directly how he feels ("I'm angry"). That's not how people talk in real life, and audiences won't buy it. Instead, characters should show how they're feeling, and the script's story and relationships should gradually emerge over the course of the screenplay.
From the John August screenwriting blog (one of my favorites):
Always ask yourself: Would the character actually say this, or is he only saying it because you need the audience to know some fact or detail? If the answer is the latter, you’re writing exposition and not dialogue. That’s not good.
August recommends listening to speakers around you for cues on how to write good dialogue.
This brings me to Elliot, who's a terrible example of "realistic-sounding" speech. Everything that comes out of his mouth is gratuitous exposition.
Here are some phrases he utters on a daily basis:
1. "I'm awake!" (yes, I figured that out when you jumped on me)
2. "I'm having fun."
3. "I'm hiding!"
That last one is both expository and self-defeating, since it usually gives away his position.
So I guess the lesson is, keep your ear out for good dialogue but avoid small children.