They try to prepare you as much as possible with all kinds of "live fire" drills. The rounds are real and so are the explosions but you know that they are not really aimed at you and you are not going to die but you do develop certain reflexes which are helpful. Often in a platoon less than 50% actually return fire and in many cases it is less.
As best I can explain something inside snaps and you decide that it is a me or him situation and I want to go home. The men you are shooting at become non-men, just objects. One becomes less than sane. It happens gradually and one ceases to think about it, as chaos become normalcy.
I posted this before but my first "shoot-out": We were walking along a trail in what was suppose to be a rear secure area so we were just diddly-boping along enjoying the scenery (dumbasses we were). Suddenly around a curve in the trail appeared a VC patrol. We all froze and just looked at one another for what seemed like a long time. Finally someone had the wit to fire a round and in a second all h*ll broke loose and we all turned and ran in the opposite diretion shooting over our shoulders. Think I ran for 5 km before I stopped for clean pants