Go to School on TNC: Fredericksburg

I'm rewatching Ken Burns' Civil War piece (For like the fifth time. It really is an incredible, if not flawless, piece. Easily the greatest doc I've ever seen.) Anyway, I was watching the chapter on the Battle Of Fredericksburg, and the point is made that the casualties were so high because, as was true for much of the Civil War, men marched in ranks directly toward enemy guns. 

It's clear that at the onset on the Civil War, firearm technology had advanced quite a bit, and thus the old tactics weren't effective. I'm more interested in how Western war was fought in the antebellum period, and the origins of the notion of massing men and advancing. (Forgive me if I'm summarizing any of this wrong. I'm a total novice.) I hear allusions to Napoleon often and how his methods were basically made obsolete by the Civil War. 

Is anyone able to speak, in concrete terms, about what this meant? How were battlefield tactics taught before the Civil War, and why were they taught that way? When did the generals begin to change and adjust? How quickly did those adjustments spread to the broader world? Any thoughts are appreciated.