I've heard ENFJs described as the type that is the quickest to flip from 0-180 as far as 'nice guy' to 'I'm kicking your ass now'. I think it's that darn Fe guided by flash-in-the-pain Ni. Fi has a more labrynthine path to take so it's more of a 'slow boil' with occasional oopsies.
I think with ENTPs (at least in the workplace) they are more like snappy piranhas (or chihuahuas) with a potentially big bite. It's always snap-snap-snap-snap and you're like "Jebus, we get it, settle down" - basically there is a pre-emptive warning and attitude or energy thrown out. With an ENFJ I think it's more a happy face that is thrown out but you can tell the lion is just being nice. Healthy ENFJs remind me of Leos in astrology, they have are magnanimous but you know they can swipe you with their claws at any time.
ENTJs combine the best/worst aspects of the two ^^ (at least in the workplace) so they can just be mean sumbitches and it's not an act.