How do you best write action scenes in a novel?

Have you read any action scenes that you really felt worked? Study those.

Generally, using shorter sentences conveys a sense of urgency and action; you rarely have time for long, drawn-out descriptions and inner meanderings when you're in a firefight, after all. You must also be clear on what's happening, so the reader isn't lost. Also, have the choreography clear in your head (draw it out, if it helps); you don't want your MC to run out of bullets in one sentence, only to be firing away in the next, for instance, or to fall off the top of a building when they were last in an alley.

Seconding what Brightdreamer said.

A couple of things to add:

- action tends to be chaotic. Your job as a writer is to convey that chaos - things happening all at the same time, things happening faster than your characters can think or react - while also making it comprehensible to the reader. Don't get hung up on describing every little thing that's happening, but make sure in streamlining your scenes that they still convey the sense of "OH MY GOD" you need.

- action is much more exciting when there are real stakes. Action for the sake of it will rapidly become boring. You need to emphasise what's at stake for the characters and why this action scene matters.

- action scenes are like any other scene in that they must reveal character and setting and progress the plot. You can show so much of a character by how they react to action scenes - don't get caught up in mechanical description of what's happening and forget to show how it affects the characters.

What Brightdreamer and onesecondglance said.

Personally I have to write action scenes in limited 3rd or 1st. I need to picture a single character's experience. But it's important not to dwell too much on internal thoughts - there have to be some, but action scenes need to keep moving. Decision points can add to tension, but you don't want to have your character musing too long over pros and cons. They have microseconds to choose a path, and you want the reader to feel the urgency while still following the logic.

I don't know your genre, but Joe Abercrombie has a reputation for excelling at battle scenes. You might have a look at his stuff (I think THE BLADE ITSELF opens with a battle), and try to break down how he's conveying the information.

Altered Carbon opens with a great action scene as well

Boggle!?!? said:

I’d like to know how to write it to be more engrossing and, I guess, ‘cool’?

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What makes action scenes truly engrossing are the underlying reasons for them: what's at stake, what's motivating the characters, what's the cost of defeat, etc.

onesecondglance said:

Altered Carbon opens with a great action scene as well

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This one doesn't count, it's a stylistic masterpiece on the basis of which people pay for movie rights after reading the second page...

BTW, two year before Altered Carbone was published, came out a "Russian Altered Carbon", called The Genome. Good stuff. Very similar themes, but through an Eastern European filter. The differences in pacing, atmosphere, and characterization are very illuminating, culture-wise and worldview-wise.

In addition to what others have said, I'd say it's important to keep track of which characters are involved in the scene, where they are in relation to one another, and what actions they're taking. If the scene is written from Character X's perspective they might not be keeping track of exactly what Character Y is doing from moment to moment, but the reader should have enough information to know that they're not just standing around waiting for their cue.