Really good question - and I suspect as you do that the answer would always depend on the situation. John Keegan once identified the 'essence' of battlefield command as what one does with the reserves, given that what happens on the front line once the fighting starts represents a kind of relinquishing of 'control'. Generals would be depending on reports from subordinates, trying to gauge which units are running out of ammunition, say, or giving signs that men are getting close to psychological and/or physical breaking points. At which time the location of the nearest fresh troops could of course mean the difference between victory or defeat. I'll have to look up where but I remember reading this was a skill cultivated by good generals known as "feeding a fight", basically trying to be the last one with fresh units to throw in.
Speaking of the "obvious," though many today tend not to believe it, the majority of commanders knew the frontal assault was not a great way to deal with a prepared position, unless meant to be a holding move to support a flanking attack. But difficulties in coordinating attacks continued to play havoc with the plans of even the best generals throughout the war.
growing experience would begin to tell combat managers what was likely to be possible, that a simple plan carried out with conviction beat the confusion of an intricate, complex So one, and when to cut losses to regroup for another day.
By no means an exhaustive, and perhaps not even a helpful, answer to a good question - but quick thoughts offered anyway.