In my Conan game (d20 3.5), I describe exactly how the foes are fighting. For example, in yesterday's game, we had a Cimmerian warrior from the PC's clan going through his deasghnath, which is the final trial a clansman endures to become a warrior. This Cimmerian, only 14 years old, entered into a ring, wearing only a loincloth, carrying a hunting spear.
His foe was a warrior of about the same age from another clan. They had caught him "spying" on the clan's village. He hung tied to a pole for three days until one of the young clansmen decided to declare the rite of deasghnath on him.
It's a fight to the death. The captured boy knows the rules. He has the same rite of passage among his own people. If the boy of the Grath clan (the captured boy) wins, he knows that honor will force the Urrogh (the PC's clan) to allow him to leave Urrogh territory in peace. If the Grath loses the fight, he won't be in any condition to leave anyway.
As GM, I played the Grath boy (his name is "Coi"). Branoc is the Urrogh warrior, played by one of the players in my group.
We started the combat with just movement--niether one wanting to give ground, each feeling the other out.
After a round or so, Branoc moved towards Coi, in a curving "horn" type pattern. Coi sidestepped to his left for several feet, always keeping his facing towards Branoc.
This we played out with the grid and miniatures (we don't use real mini's--just makers like tacs, dice, pencil marks, and buttons and stuff).
Then, combat began.
Coi won iniative.
I described him as leaping left, rushing two or three steps, then jabbing his spear straight into Branoc's ribs.
In our game, we use Active Defense where people roll for defense instead of the attacker rolling against an AC number. It averages out to about the same, but we find the optional rule quite exciting because you never really know what's going to happen. An attacker could roll a 3 and still hit. I've seen it happen.
So, the player playing Branoc said, "I'll parry the strike." (In the Conan game, you can Parry or Dodge a blow, so you've got two types of defense instead of a standard AC.).
Like you, I wanted more than that. I wanted the player to describe his action, so what I did was have him clarify.
"The Grath is shoving his spear straight under your right arm pit. Be more specific. How are you parrying such a blow?"
Once I asked him that, he showed me how Branoc's hands would change grip and twirl the spear almost like a propeller, batting away his foe's spear.
When he described it properly, I rolled the Grath's attack, and he rolled Branoc's defense.
The dice showed us that Branoc was successful.
"Indeed, the Grath rebounds from you shoving his spear away." I said, showing him how the Grath stumbled back a step. Now, it was Branoc's turn to act, so I asked the player, "You're standing there like this, holding the spear like this....(showing him how the scene played out)....how are you going to recover and make an attack?"
The player thought a moment, then gave me a plausible move. "I bend one knee, angling my body," he says....and I noticed that he switched from using third person Branoc-does-this to describing the character's actions in first person. I knew that was a good sign. "And, use my left hand to swing the butt of the spear up into the Grath's belly with a low strike."
I thought that was great. He rolled his attack...and it was a "1". Oh noooo!
In our game, if you roll a "1" on your attack, you've opened yourself up to a conditional Attack of Opportunity from your foe. The condition is that the attack cannot be lethal. You foe can throw a punch, head-butt you, elbow you, knee your or kick you. He can use a Combat Manevuer such as an attempt to trip you or a feint (as long as the "feint" part of the maneuver comes on this attack while the acutal attack comes on the foes turn). He can even attempt to grapple. If a nonlethal attack would normall call for an attack of opportunity by the target (such as when beginning a grapple or landing a punch on your target), the character that rolled the "1" does not get the AoO in this case--making it a good opportunity to start one of those nonlethal maneuvers.
I decided that the Grath would attempt to Sunder Branoc's weapon. The Sunder was successful, but the Grath only did 1 point of damage to Branoc's spear (the spear still had 3 points left).
Here, I described how, as Branoc shoved in low with his spear, Coi changed his grip and swund downward, in a semi-circle, as you would a bat, and connected with the spear. Branoc shaw a chip of the haft fall off and could feel, under his finger tips, a crack running up and down the shaft now.
The player was doing two attacks, holding the spear as one would a quarterstaff (I didn't say that above. I gave him the two attacks but each did 1d4 while the spear usually does 1d8).
With Branoc's second swing, he came around with his right hand, according to the player's description, using all his might to slam into Coi's jaw.
The entire movement was: Branoc bent his left knee and did an upper cut motion with the spear into Coi's gut. This is where the "1" was rolled. Coi shifted his weight, changed his grip, and swung the haft of his spear into that of Branoc. This was the sunder attempt.
Branoc then stood high and put all his power into the right hand blow. I missed Coi's Parry, so I described how he fumbled with the spear and stumbled backwards trying to recover the weapon after the sunder attempt.
Branoc's second attack was the blow that killed Coi. And, I could tell the player was "into it" as he described how "he" could feel his heart beating, his chest visibly heaving, as he stepped over the downed Grath, raised his spear high, and with both hands shoved it into the Grath, stapleing him to the ground.
I then described to the player how the Grath shuddered a bit then lay still, air and blood gurgling from his mouth.
I had another NPC throw a hatched into the "ring". The player saw it and knew what to do.
He said, "I grab the Grath by the hair, and I hold the hatchet high. One, two, three swings, and I pull his head from his body. Blood saturates the spot. Then, I hold it high for all to see."
It was a cool moment in our game...
But, you asked how I got a player to roleplay the combat. He's an old AD&D 1E grognard, too.
Well....that's how I did it.