As with any writing project, it is important for the writing style to remain consistent throughout the project and for each scene to keep a foot in the realistic world, no matter the actual level of realism in the world and story.
Consistency in Style
When writing an action scene, there will be some changes in style depending on the scene elements involved, but the scene should not feel like someone else wrote it because of drastic alterations to the writing style. Maintain the style used throughout the story with small changes to increase the dramatic tension (more high emotion descriptors), change the pacing (shorter, more urgent sentences), remove any comprehension barriers to move the reader through the scene smoothly (avoid using complex or unfamiliar words that might trip the reader up), and so on.
Actions scenes will necessarily have slightly different language and structure than exposition, but it should still mesh with the rest of the story. Sudden, drastic changes pull the reader out of the story. If you tend to include detailed description in other scenes, be more sparing with such language during action scenes, but don’t necessarily cut it all out completely. If movement and technique are important to the fighting style and have been included in less action-based scenes, include some of that in an action scene, but don’t overload the readers or sacrifice the flow of the scene in order to include it.
The characters should sound the same when it comes to spoken dialogue or thoughts, as well. Characters involved in an action scene should not suddenly sound like anime characters, not should they start spouting Shakespearean-style soliloquies. Internal dialogue should not vary widely from the character’s norm. If the character is highly internal and previous scenes have included a significant amount of internal thought, include some in action scenes where appropriate but keep in mind the pacing of the scene. Slight shifts to mimic panic or desperation work well, but complete changes make the character feel like a different person.
Writing style should stay consistent. Don’t suddenly change stylistic elements like syntax, tone, mood, exposition, or narration. The scene will feel out of place and confuse the reader.
Action scenes aren’t technically realistic in many cases. Characters don’t take as much damage as they should or are able to maintain stamina way longer than is reasonable, or have skills outside the normal human range. Even so, actions scenes should maintain an air of realism to keep the scenes grounded, even when set in a completely fantastical world.
Action scenes must be real enough to convince readers to suspend their disbelief. The action taking place should make sense for the world and the characters. There may be surprises, of course, but a piece of action should not make the reader stop and question the reality of what happened. This halts the scene and can frustrate the reader.
Avoid impossible abilities to keep characters from coming off as indestructible. Whether a completely realistic world or a non-realistic one, there are still rules the character must comply with and work within. An ability or action that breaks rules with no explanation not only stops reading progress, it risks alienating the reader.
Use realistic (for the world) healing times and make injuries impact future events. Broken bones take weeks, if not months, to heal without magical or advanced scientific methods. Injuries can be great barriers to a character reaching a goal if used properly. If you don’t want a character out of commission for a long period of time, choose a more appropriate injury for what you have planned.
Research weapons and how they work, including the sound, feel, power, weight, construction, impact, etc. Different types of guns feel very different to fire, and metal swords are much heavier than what is typically portrayed, with the exception of fencing and dueling types of blades. You want to be able to accurately describe the action using all five senses.
Watch videos when possible to better describe movements and reactions of people engaging in the type of action you’re attempting to describe. Ask people who participate in the activities involved when possible to learn more about the feel of the movement and the difficulty of responding when in a high-intensity situation. This will help you to be more consistent in writing how characters fight, as well. Learn the proper terminology and work in enough to sound realistic without overwhelming the reader. Experts are usually very willing to help with this.