Raindance.com says....

 

There are 7 rules you should remember when writing a short film:

1. The Shorter the Better

A short film can be anything from fifteen seconds to forty five minutes in length. Make your short film script as short as possible because the shorter the short, the less costly it will be to produce. It has to be cheap but shouldn’t LOOK cheap! 

 

2. Keep the Practicalities of Writing in Mind

The great thing about shorts is that they can be anything since you don't have studio execs on your tail. However, don’t discard the practicalities of writing your script.If you only have access to modest resources, think small.

 

3. Make It Visual

“Film is a visual medium”. “Show, don’t tell.” Those are the golden rules of screenwriting the gurus keep telling us. Yet it’s astonishing to see how talky most scripts are.When you make a short film, economy is everything. Create visual backstories for your characters. Externalize through visual images their temperament, their profession, their status, etc.

 

4. Find Single Moments

The best short films are often a single moment that is played out, but one that has a story at its heart. A conflict that has to be resolved, where there’s a dilemma at stake and a choice that the protagonist has to make. Strive to add a deadline, or ticking clock. 

 

5. Tell a Story

You should always try to tell a compelling story. Beware of ideas that are concept-driven or just aim at breaking all the rules for the sake of breaking rules. Short films are a great opportunity to push the boundaries of what cinematic storytelling can do, yet they must still engage your audience emotionally. 

 

6. Engage the Reader

Since you have so little time to make an impression the impact of page one is crucial, just as it is crucial to hook the reader in the first 10 pages of a feature length script. The ending is also essential as it’s rare to truly feel moved at the end of a short, so work towards a meaningful, satisfying ending.

 

7. Beware of Cliches

There are many clichés in short films, and much navel gazing. Write what you’re familiar with and what resonates with you rather than writing something you borrowed from other films. Don’t shy away from small stories, short formats are the perfect vehicle for them and you won’t often get the opportunity to tell small stories as a professional writer.