Dregs are the most extreme case of this. You charge they run. You pull back they follow if not charge. The higher their level in relation to yours makes them bolder, and the inverse is also true. Named captains do not care what your level is you chase they charge, wizards tend to be a little more timid. If you do not chase they hide and regenerate shield. The shield re-gen trait is true for all shielded enemies, once it's down they try to hide.