1) Never explain something you can dramatize

Master storytellers never explain. They do the hard, painfully creative thing — they dramatize. Audiences are rarely interested, and certainly never convinced, when forced to listen to the discussion of ideas. Dialogue, the natural talk of characters pursuing desire, is not a platform for the filmmaker’s philosophy. Explanations of authorial ideas, whether in dialogue or narration, seriously diminish a film’s quality. A great story authenticates its ideas solely within the dynamics of its events; failure to express a view through the pure, honest consequences of human choice and action is a creative defeat no amount of clever language can salvage.”
–Robert McKee, Story

2) We must learn about characters by what they do

The truth is that movies are so much about what happens that we must learn about character by what they do, not by what they say. As in Life, character is revealed by action taken, not by words spoken. And in a good movie, information doesn’t come out in dialogue, it comes out in the verve and forward motion of the story. You must get out all your wonderful plot and backstory on the fly, or better yet, not at all. you should be more concerned with what’s happening now than what happened before the story started. So when you find yourself drifting into talking the plot, don’t. And when you think you’re talking too much: Show, don’t tell.
–Blake Snyder, Save the Cat!

3) Voice-over and flashbacks tend to be overused

Let exposition emerge naturally in conversations…unless you are writing a broad comedy. Obvious exposition includes voiced-over narration that adds little to what we already see on the movie screen and flashbacks that stop the momentum of the movie.
–Dave Trottier, “My Favorite Flubs”, Script Magazine, October 21, 2010

4) The present is better than the past for revealing character

No matter how good the scenes from the past are, they tend to not have the same sense of emotional import. They tend to play more as information about how the character got to their current place. This can be important to do, and it’s certainly better to see these things in dramatic scenes than hear them described in dialogue, but what’s even better is to see dramatic scenes in the present, when the outcome is still unknown.
–Erik Bork, “The Problem with Flashbacks,” May 24, 2015

5) Undramatized exposition is boring in any light

“It does the writer no good to write an exposition-filled scene in which nothing changes, then set it in a garden at sundown, thinking the golden mood will carry the days. All the writer has done is dump weak writing on the shoulders of the director and cast.  Undramatized exposition is boring in any light. Film is not about decorative photography.”
–Robert McKee, Story

6) When exposition is necessary, keep it short

Exposition is necessary at the start of virtually every film. The rules: Keep it short, present it in a fresh manner, and avoid making more of it than it is worth.
–Richard Walter, Essentials of Screenwriting