A fight scene is put into the plot not only to liven up the action but also to move the plot forward. Figure out what is at stake for the viewpoint character and the other characters. Make the possible results of the fight, beyond dying, as dangerous as getting killed.
This is the beginning of a fight scene in STAR-CROSSED. Kellen is being transported by two soldiers to his first owner and a life as a sex slave, and one decides to try him herself.
When she invaded his mouth, Kellen heaved with nausea. For the first time, he understood the violation of rape. He fell backwards onto the floorboard with her on top of him. She weighed more than he did. Her hand slid into his pants.
As she touched him, he realized that it would be die or escape. No middle ground of surviving in the harem was acceptable to him. He hit her then, a killing blow to the throat. She gurgled and arced like a woman in orgasm and went limp.
For Kellen, at this moment, death is preferable to what is in store for him, and escape or death are his only options, and the reader knows this, too.
The fight should also offer at least one or two pieces of the viewpoint character's emotional puzzle to the reader as well as telling the reader something about the opponent.
In this scene from THE ONCE AND FUTURE QUEEN, I wanted to show my hero Val's skill at stopping a fight, not in winning one. He's facing his rival for the Queen in an exhibition match that quickly turns real. Prince Gregory also shows his true nature in this fight.
During the first blows, Val concentrated on his defense and let his muscles settle into the rhythm of swordplay.
After several minutes of attempting to get past his defenses, Prince Gregory began to batter at him as if to pound him into the ground. The prince had expected a quick defeat and easy humiliation, not an equal opponent, and his simmering anger about Fira now boiled. He wouldn't be content with pretend wounds and victory; he was out for blood.
The crowd, who had chattered and cheered their local favorite, became completely silent, and the air rang with the tintinnabulation of the singing blades and the hoarse rasp of both fighters' breathes.
Val thought desperately for a way out of the mess.
Gregory's weapon slipped past his defenses and slashed toward his throat. Val dodged, laughing as if having a marvelous time. He praised loudly, "A wonderful strategy."
When Gregory slashed backhanded in a return blow, Val thrust his blade vertically and caught it before it cut him in half. "Excellent. Excellent. You're one of the finest swordsmen I've ever seen."
Gregory blinked as if coming out of a daze but continued to go for blood.
Val laughed and spouted praise for almost a minute before the prince's attack began to ease in its brutality. Their weapons caught each other high in the air, and they stood belly to belly, face to face.
Gregory whispered, "What the hell are you doing?"
"Dying is a messy, bloody, ugly thing. I don't want to kill you in front of Fira, and I don't particularly want to die in front of her either. Where I come from that's not acceptable. If we must fight, we do it without a female audience."
The boy glanced toward Fira who stood white and silent, her hands clinched in painful distress. "I had forgotten...." He danced away, bringing his sword forward. "Another time then."