Fight scenes can be particularly difficult to write because there runs a high risk of repetitive action: punch, punch, kick, kick. That’s not to say that all fight scenes are like this, merely that it’s a risk. Another risk is a character overthinking an action. Do we need to know why they’re going to do something or what they are going to do? Are they pinned down and debating their escape? Or are they thinking quickly and start to move so the reader wonders how they will get out of it until they do? Then we have sentence length to consider, long descriptive sentences usually slow action down while short ones speed it up. A well written fight scene can look easy when we read one but when we consider the questions that go into writing one it gets a lot more complicated.
Let’s begin with the question which seems the simplest to answer. If we’re writing a quick moving action scene then the way to go seems to be short snappy sentences. For example:
I hit him really hard.
I hit him.
I hit him, hard.
I offer two options because the hardness of the hit might be significant. It might seem like an odd thing to say but hitting someone doesn’t automatically imply a hard hit. We may have a hardened fighter, every blow is direct and intentional, or we may have the novice pushed to extremes by circumstance and their first blow is weaker, the act is unfamiliar and they’re afraid. So, perhaps, for the second example they have struck their attacker once but it wasn’t hard enough, the second time their more confident and more desperate and they strike harder. We may even have a character with a tendency to pull their punches, for instance, Superman probably wouldn’t hit a human criminal at full power. In this case the hardness of a blow takes on another meaning.
It can come down to how fast we want the scene to move and how long we want it to be. Sometimes less is more and a fight scene can be very effective short and with minimal description. Other times we may want it to be longer, perhaps a battle scene or a character is struggling or trying to escape. It is not a universal that if we want effective prose than shorter is better, it depends on circumstance. With fight scenes it can be particularly important to remember the question ‘Do I need this here?’ because too much detail can muddle the scene and slow it down. An instance may be if there was a paragraph of description in between two punches that are supposed to be in quick succession would give the impression of a pause in-between; while short sharp sentences where movement is meant to be slow would imply speed instead.
I can’t give you rules for writing fight scenes, as I said it all comes down to what you’re trying to do. In the end experimentation is best to find out what works best for what you’re trying to write.