1, 2, 3...Roll Camera
Most screenwriters and filmmakers use a simple 3-act story arc. Their movies have a 1-2-3 punch of a set-up, confrontation and resolution, according to a formula explained by screenwriting gurus Robert McKee and Syd Field. Roughly divide your two-hour story into four sections and three acts. Act one, about 30 minutes long, contains the inciting incident that starts the action barreling along to its inevitable conclusion. Near the end of act one, the hero or main character chooses to engage with the problem or challenge at the moment of a reversal that sends the plot off in a new direction. In act two -- about 60 minutes of the film -- the action builds towards a mid-point and then spirals to another reversal, the second main plot point. This hijacks the action and pitches it toward the climax -- act three, all of the remaining movie.
Timing is Everything
The inciting incident that sends the story off on its real journey happens near the top of the film, or you have a box-office flop. You either need a short time to establish the affected character, cement interest in or sympathy for her, and then upend her world, or you need such a decisive and dramatic opening that the audience is glued to its synthetic velvet seats and forgets about the popcorn. In "Save the Cat," screenwriter Blake Snyder explains that audience empathy is essential but the protagonist's entire back story is not. You could have the tough guy save a starving kitten in an unwitnessed tender moment, so his later vigilante rampage doesn't alienate the audience. The opening-scene terrifying curl of a tsunami behind a laughing couple at a resort is the set-up for the character-building struggle of the lone survivor.
Desire Plus Obstacle Equals Conflict
Know your players before you set pen to page. Movies are not travelogues or educational aids to teach tadpole development. They are about life and death events, epiphanies and emotional arcs of characters, whether those characters are people or penguins. You don't get to dictate hair color and casting; you do have to invent a believable and compelling hero, villain, sidekick. Then you have to march that player -- or those players -- through the story advances and reversals that sweep the audience along. Relate the action in present tense, active voice: the character wants something very badly; a daunting obstacle interferes; the conflict engages character and audience until the resolution. Use dialog to reveal character. It has to sound natural, which means it is an artifice because real conversation is coma-inducing. Read your written dialogue aloud to see how it plays as spoken, not written, words.
Function Dictates Form
Your script is an outline, in some ways the bare bones of the story, ready to be fleshed out by the artists and technicians who create the final product. A film script contains everything that will end up on the screen -- just in a haiku-like form with a lot of white space. If you don't know the form, your script dies unread. Screenwriting software makes script formatting easy but it's useful to know basic form. For example: location and time are capitalized and centered on a separate line -- EXT. STANDING STONES ON HILLTOP - NIGHT. Scene descriptions run margin to margin, left to right. A character's name is all caps when first introduced and is written uppercase and centered on a separate line over centered dialogue. Audible sounds are capitalized. Read a few shooting scripts to get a feel for how to write yours -- and for what to leave out.