For one, it's not quite as chaotic as some Hollywood depictions show it. Most armies will do their best to keep a semblance of order (well, insofar as the Greeks could do it) and to keep the enemy in front of them, specifically to avoid potential chaos. I commend to your attention these following previous posts:

• u/Iphikrates shows how hoplites fought and stayed together in battle;
• u/Celebreth looks at how the Romans fought and possible means of identification;
• and for contrast, and to show just how little really changes when it's heavy infantry clashing in close combat, here's u/400_Rabbits looking at how they did it in Mesoamerica.