Develop a plot. Action movie plots don't have to be too sophisticated, but they do have to go for the gut. Basic action plots include the hero saving the world (or some microcosm of the world), saving his family, extracting revenge on the villains who hurt his family, stopping a large-scale conspiracy or finding a treasure or reward.
Draft your characters. Action movie heroes tend to be strong male or female characters with the ability to endure against all odds and a special skill. Action movie villains tend to be rough-and-tumble gang leaders, foreign terrorists, sophisticated corporate executives or crime lords and the occasional alien. You should always include a civilian in danger (often the love interest), a wise-cracking sidekick and a bureaucrat who frustrates the hero with rules and regulations.
Build your settings. Action movies need big sets where large-scale disasters can unfold. It's okay to destroy the hero's home or office, but you should include plenty of exterior shots with large explosions and vehicle chases: shipyards, downtown office buildings, airliners, national monuments.
Write an outline. Action movies follow three simple acts: the hero experiences a call to action after an act of villainy, the hero mounts a first offense only to discover a major obstacle, the hero engages in a final confrontation where he prevails (usually going one-on-one against his opponent). Look for every opportunity to throw in hand-to-hand combat, a shootout, a street brawl, vehicle chases, physically demanding stunts--basically as much abuse to the hero as possible and as many explosions as you can justify. Throw in at least one plot twist (the love interest or sidekick sells the hero out, the package the hero has to intercept is actually a bioweapon).
Document your effects visually. Action movies are about seeing, not thinking. Don't simply write, "Action: Explosion"; imagine the camera angle, the type of explosion and what happens to the characters in the surrounding moments. Describe your effect with as much detail as possible to help the director and storyboard artists move the scene from script to frame.
Finish your script. Follow screenplay format to the letter, and figure a page of screenplay as 1 minute of film time. Action movies typically last 90 minutes, although big-budget films can run to 145 minutes. Don't pad the script with scenes, however; if a scene doesn't move the action forward, leave it out.