Action scenes are a crucial part of narration in a story. They aren't always sword fights or full-fledged battles, but regardless, the nuts and bolts matter. Whether the scene is a small plot point in the story arc or a large one, the craft of writing one should be done with care; action scenes can be tricky. So my advice is to focus on two main areas when getting down to the nitty-gritty: the writing structure itself and the minute details that make up the structure. Each contributes to the scene to make it more intense and realistic.
Writing Structure
First, pick up the pace. Use shorter dialogue and sentences to move things along. However, try to harmonize this with natural flow and keep things in "real time." By that, I mean describe the actions of each passing minute. A greater abundance of details in those short sentences lets the reader know that this scene is an important one that will likely have a long-term affect on the plot. Another trick to speeding things up is to make your characters react with their gut more than their brain; the act of deciding quickly without dwelling on future consequences brings a sense of urgency to the scene.
Action scenes are one of the few places where readers really expect and appreciate extra tension and big reactions. In other words, drama. Drama is one of those touchy elements that I'm not overly fond of personally. Too much drama throughout a book can be rather annoying, and it tends to make the book less believable. However, when paired with a specific action scene, drama has the power to intensify the situation and increase the reaction of both characters and readers. Adding unexpected consequences or extra conflict will heighten the drama in the scene and can increase the stakes, making the impending actions of the characters involved pertinent to the outcome of the plot.
Close calls will enhance that drama. They cause characters (and readers) to hold their breath and hope for the best. They can be used to build tension or to introduce new conflict. For instance, if one character is running from another and both have guns, a few near misses will likely be exchanged before someone is hit. But once one of the characters is wounded, the whole dynamic of the chase changes. If the pursuer is injured, the person he is chasing might very well get away, amplifying the stress of the situation and developing the plot further. If the character fleeing is the one injured, the reader will suddenly become tense alongside the character.
Details
Details help polish a well-written action scene. They are the last-minute touch that really dresses up good writing. Strong verbs are one of those details. The stronger the verb, the stronger the action that is conveyed. If you see a weak verb anywhere in an action scene (i.e. went), replace it with a stronger one (i.e. strutted).
The five senses are another great asset. They increase tension and urgency. When a description of those senses comes into play during an action scene, a short wave of slow motion can be perceived. This type of effect is quite simple to achieve in movies but is a bit trickier in writing. Adjectives as well as a few well-placed lengthy sentences can be used to achieve slow motion.
General Tips
If you're still struggling with perfecting that action scene, try these tips:
1. Act out the scene to get a handle on body movements before writing them.
2. Research any weapons being used in the scene and how they are used.
3. Study other writers, particularly ones that are known for their craft of writing great action scenes.
Overall, use action scenes sparingly and balance them out with other types of scenes. A nice balance of scene descriptions and effective pacing throughout the story can bring an intensity and power to the action scenes.