Writing a clear, fast action scene
Action scenes look like the easiest to write, because "a lot happens". In fact they are among the hardest to read: one ambiguous pronoun or a room never described and the reader stops seeing, rereads, loses the rhythm. Action on the page isn't a film: the reader has no images, only your sentences, one after another.
First the map, then the movement
Nobody can follow a fight in a place they don't know. The right moment to describe the space is before the action begins: the narrow staircase, the open window, the table between the two of them. A few coordinates, chosen because they'll matter.
Once the scene starts there's no time left to describe, and every line of setting dropped in halfway slows you down exactly where you should speed up.
One action per sentence
The long sentence with three subordinate clauses works for reflection, not for a punch. In an action scene, one sentence carries one action:
He grabbed the handle. The door didn't move. Behind him, footsteps.
Short sentences, concrete verbs, clear subjects. Not all identical, though: a slightly longer sentence now and then gives breath and makes the next one snap.
Who does what: watch the pronouns
Two characters of the same gender fighting are fertile ground for confusion: "he pushed him and he fell". Use names more often than you would elsewhere, or distinguishing features ("the taller one", "the man in the coat"), until the reader has no doubt.
What's at stake
A sequence of blows with nothing at stake is choreography, and choreography gets boring fast. The reader needs to know what happens if the character loses: their life, the document, the person they're protecting, their cover. Tension comes from there, not from the number of moves.
Even a very short action holds if the stakes are clear; a ten-page battle collapses if they aren't.
Sensations, not explanations
In the thick of action a character doesn't think in paragraphs. A few strong perceptions convey urgency better than any analysis: the taste of blood, a noise too close, a hand slipping. Reflections, if needed, come afterwards, when the scene slows down.
After the action, the reaction
An action scene doesn't end with the last blow. It needs a moment, however short, in which the character registers what happened: breath, damage, the decision about what to do now. That's where action becomes story, because it changes something in the person who lived it.
Frequently asked questions
How much technical detail does a fight or a shootout need?
As much as the character knows and notices. An expert sees precise details, a layperson only perceives chaos. Technical detail is believable when it fits the point of view, not when it's abundant.
Don't short sentences make the prose choppy?
Only if they're all the same and go on too long. Alternate them with some fuller sentences and use them only at the heart of the scene: outside the action, the rhythm returns to the book's normal pace.
How can I tell whether the scene is clear?
Reread it after a few days and try sketching on paper where everyone is in each paragraph. If at some point you don't know where to put someone, the reader doesn't know either.