I'd like to see this done so that upon joining an army you are made member of a unit, say, 5-10 men with names and personalities and then the lord sends your unit on missions like foraging, reconnaissance, tax collection, bandit hunting or just guarding a bridge. Even fairly mundane military tasks can serve as the setting for an interesting quest with RP possibilities. If the lord's party actually goes into action then you'd fight with the guys in your unit, who might get hurt or die. If the lord's army is defeated, you could choose to desert with your remaining friends, who then become your companions.