The thing is, you and your mates are standing there with your pointy sticks, afraid to charge. If me and my mates start running at you with our pointy sticks, what's are you going to think? They must be mad! Your flight-or-flight response starts leaning heavily towards 'flight'. Meanwhile, we've got our adrenaline up, and we're well into 'fight'. If one of you flees, the others soon follow. And when you run in an ancient battle, your chances of dying go up exponentially. It's counter-intuitive to you, but in many cases if you don't want to die, you need to charge. Psychologically, receiving a charge while holding your ground is one of the harder things for a soldier to do. This is where training and discipline come in.

Your time period is pretty broad. What a battle would be like varies, depending on the time period, who was fighting, etc. I'll try to cover the ancient period.

Throughout much of the ancient period in Europe, most armies depended on some form of shield wall. Men are lined up together, usually 6-10 men deep, so that the men on your right and left protect you and you are only left fighting the man directly in front of you. The Greeks used the tightest formation, shoulder to shoulder with the man next to them, and partially covered by the shield of the man to their right. If you watch the very first part of the very first battle in 300, that's about as close as you get in Hollywood to what it looked like. (The rest of the movie goes 'moshpit' pretty quickly) The Greek formation, however, was inflexible and hard to maneuver, so the Romans loosened things up a bit by breaking up the big mass of men (phalanx) into smaller groups that could move relative to one another. The Celts and Germans also formed a shield wall in their own fashion.

There's a lot of speculation about how ancient battles actually progressed. The battle between the two front lines probably consisted of a lot of ebb and flow, bits of intense combat and bits where the two sides separated, catching their breath and recovering their courage. This continued until one side broke and ran, or at least lost cohesion. This could occur in the first minute, or it could take hours. As soon as one side started to break formation, gaps would appear, and there would occur situations where one man would be facing two or three opponents, and he would quickly die or flee. The gap would then rapidly widen. Once this occurred, the side that lost cohesion would quickly break and run. The side that fled would then be set upon by the winner's cavalry. For your Hollywood moshpit to occur, both sides would have had to lose cohesion at the same time. Not to say this didn't happen. The battle at Kalkriese between the Germans and Romans was probably pretty chaotic. The Romans were never able to get in proper formation due to surprise and terrain (as the Germans intended), so there would have been many isolated groups trying to fight.