Well, eye contact, body language, speech patterns etc.  But let’s try a thought experiment.

Think of a Bad Idea.  Something with high potential for disaster that you would rather not be involved in.

Imagine Kristen Stewart coming to you and presenting you with this Bad Idea.  Do you feel comfortable telling Kristen that you would rather not, because x, y, z?  I would!  I would be like, “Kristen, you do not really seem super enthusiastic about this Bad Idea and I’m not sure why I would want to do it with you.  It is pretty weird that you want to do this – maybe we could go do anything else.”  Kristen would probably be like, “Yeah, okay, I see your point,” and that would be that.

Now imagine Jennifer Lawrence coming to you with the same Bad Idea.  Imagine her continuing to tell you why the Bad Idea is great until she finds the one redeeming feature of the Bad Idea that you can’t argue with.  Imagine her enthusiasm growing as she finds all the related good things the Bad Idea has to offer.  There’s no pressure – she just thinks the Bad Idea is hilarious and wants to have a good time with you and some mutual friends!  Is it as easy to wipe the smile off her face?

ENFPs have Ne as a first function.  They love possibilities and sharing them with others above all other things.  INFPs will consider the same possibilities, but won’t share or try to make most of them happen.  With Fi as our first function, we want emotional stability/honesty first.  I think there are probably many people who live in fear of the day an ENFP drags them out of their comfort zone – the things they want us to do aren’t things we’d do for anyone else, but we do them because ENFPs are so unbelievably difficult to turn down.

Fi makes me incredibly sensitive to group dynamics – for example, I always know who’s in charge and if they have a good hold on their leadership, or if they’re losing it to other people.  (When it’s not balanced, there goes my emotional stability.)  Jennifer is sort of the bright light in a group – people turn towards her, expecting to be entertained.  Kristen is present but independent – she doesn’t necessarily play along with whatever assumed rules are in place (although she’s reactive, not active about breaking them), so people tend to give her a lot of space and treat her as kind of a curiosity (other introverts may recognize the uncomfortable “an introvert is speaking!” silence).