The admonition to screenwriters to "show, don't tell" is about as old as the first time a meddling producer says, "I have notes." Admittedly, I give this note at least five times a week too, but what does it mean?
Usually, it means that the dialogue is spelling out something that is either self-evident, or could be shown visually. Ask yourself - which is more interesting to watch:
1) a character calmly walking into a room, taking a deep breath, then smashing his priceless Ming vase on a destructive impulse.
2) a character walking into a room and snarling at his friend, "I'm madder than I've ever been before and I could smash my most prized possession and not even care."
Pencils down. That was an easy one.
If you want to see an incredible example of "Show, don't tell," check out Pixar's UP. There's a wonderful sequence that fills in all of the lead character's history with his wife in less than five minutes. There's virtually no dialogue in this sequence after we meet them as kids, see them marry, see their hopes and dreams for the future, even as their dream to explore gets put on hold. Then we see her take ill and go to the hospital, as well as her implied death - all of it done without dialogue and all of it done with some of the most simple of slices of life.
Another good example happens a few moments after that. We've already been tipped off that developers want the land our hero, Carl's, house is on, but he won't sell. When one construction worker accidentally damages the mailbox outside the house (the sentimental value of which having been set up in the aforementioned montage.) Carl attacks the worker with his cane, an act he immediately regrets.
In the following shots we see:
- the developer react with interest, looking like the cat who ate the canary.
- a crowd grow outside as Carl runs inside.
- From Carl's point of view, looking out, we see people telling the cops what happened.
Then, we're shown Carl waiting outside a courtroom, holding a court summons. He walks into the courtroom. We don't see the hearing. Instead the film dissolves to a clearly defeated Carl being dropped off outside his house by a cop. His shoulders are slumped in defeat, and it's clear what must have happened.
Then comes a line that I would bet was added late in the process by an exec concerned that the audience wouldn't make the correct inference: "I don't think you're a public menace. The car will come by to pick you up tomorrow," says the cop as she hands him a brochure for a senior living center. My feeling is that Carl's defeat is pretty clearly communicated by the visuals (kudos to the animators who worked on his body language.)
In any event, a good exercise is to go through your script and see how many lines you can eliminate by replacing them with some bit of physical acting or visual action.