If you imagine 20,000 people swinging at 20,000 people it is obvious battles would have been finished very quickly, even with multiple lines of depth. Battles could last for many hours. Charges were common as an attempt to incite opponent to flee. But self-preservation is deeply imprinted and running into a line of swords/axes/pikes is not something an average person would do in any period. So battles had a lot of standing on a safe distance and shouting at the enemy, with very valued "hero" personalities leading occasional attacks on enemy lines in an attempt to break them. This varied greatly depending on the experience and professionalism of combatants, for example in later periods pikemen charges were one of the most devastating and often used tactics, but it took experienced soldiers to perform correctly.