Whether you’re writing action and adventure stories, or any other genre, at some stage you might decide to put your characters into an action sequence. Action can enliven a plot and add challenges for the characters that stretch their skills and character development.
Style
Action scenes have a quick pace, which you can achieve by starting with fairly short, snappy sentences. Once the characters are fully involved in the action, switch to longer, run-on sentences that suggest the actions running into each other. Pause when your characters pause, to maintain the illusion.
Planning
Action sequences in writing need to be choreographed just like movie action scenes. If you freewrite, you will have to break down your action scenes when you edit your first draft and check that the steps flow logically.
Ideally, you want to stop and imagine the action steps your characters take before you write them, so that you can see the scene as you capture it. This avoids illogical movements or gaps in the action where your reader could get stuck trying to work out how you moved your character from one location to another more convenient point.
Action and reaction
Every story is made up of a series of events plus the characters’ reactions to each event, which in turn causes new events to occur.
Check that your character’s various reactions happen in a natural order, though. It’s easy for a writer to get wrapped up in an action scene and present a series of reactions out of order because it fits in with the plot strand.
In the midst of a suspense or action scene, the character will first experience an adrenaline rush (an instinctual response to stimuli he has picked up before he’s even seen or realised the danger).
Next, his reflexes kick in – more instinctual behaviour, that is based on whether he has prior experience with dangerous situations, or whether this is a first. This is the “fight or flight” reflex.
Only then does the character begin processing thoughts about the event, that might encompass decision making, panic, fear, excitement, determination, etc. Finally, the thought process leads to its own series of logic-dictated action, which may enhance or contradict the instinctive reaction that has already begun. For a first-timer, the initial instinct might be “run”, but his logic might override that because his wife is in danger. The result is a momentary hesitation as he switches from “about to turn and run” to “get in there and save her”.
Don’t be too specific
Don’t commit yourself to left and right if you don’t need to. There’s no need to specify that a character picks up a knife with his right hand while the other character runs through a door on his left, unless it is important to the plot (eg, the character is left-handed and injured). Details like left and right make the reader stop to check their orientation and that breaks the illusion of the action scene. Give just enough detail that the reader can imagine the scene without being “wrong”.
For more help with crafting your action sequences, try Conflict, Action and Suspense by William Noble.
If you're a screenwriter, you might find Writing the Action Adventure Film : The Moment of Truth by Neill Hicks useful.

