"The camera can't see what is going on in a character's head," he says. "The story must be told through action, dialogue and imagery."
I'm not a script writer (yet), but I recognized this as good advice for any writer wanting to engage their readers. What your character is thinking adds very little to moving the story forward, except in an explanatory fashion. Far better for your character to speak and act to create action and reaction.
Another example of show vs. tell: Several years ago, I took a trip to Kauai which I described as "one of the most beautiful places in the world".
While this sentence created a little bit of interest, I was only telling my readers this was a beautiful place. I wasn't showing them anything. This wasn't very compelling. I needed to use imagery to place my readers into the scene by engaging their senses to captivate their interest.
Here is a much more interesting way to describe Kauai:
"I walked off the boat into a shock of humidity; a wet blanket infused with the scent of a wild fruit. A thick tangle of emerald green jungle lay before me, sweeping upwards into the serrated pinnacles above the Na Pali coastline."
This sentence puts the reader into the scene to feel, smell and see why this is, indeed, one of the most beautiful spots on the planet.
Never forget your readers are honouring you with their time and attention. Honour them back with a feast for their senses. Give them something meaty and compelling so they come away feeling like they've been there!