Tactics as in actual battlefield fighting.
Documentaries which show reenactments tend to show the Romans as highly disciplined when fighting. Reenactments of Medieval battles are shown as being brawls.
How did tactics evolve between the 2 periods? In what ways was it similar? In what ways was it different? Was there a regression in anyway?
I would just like to point out that the idea of medieval battles being uncultured brawls is misleading. We have numerous treatises from the medieval period that showcases fighting techniques (such as [Fior di Battaglia](http://www.wiktenauer.com/wiki/1400s_-Fior_di_Battaglia(MS_Ludwig_XV_13)), Gladiatoria and the Tower fechtbuch. These are highly sophisticated methods of fighting that demonstrates a very deep understanding of both biomechanics and universal martial principles.
Since you are interested in battlefield fighting, here is a short video that showcases what late medieval plate armour is actually capable of.