I'd like to understand this.
From what I understand, I think that INFPs are more likely to be afraid of all the possible things that might happen. For me, I kind of expect that I know how things will play out simply by combining a lot of unrelated factors to reach a conclusion. Which is something I do kind of unconsciously. Combine that with anxiety, and I'm constantly assuming that I know how things are and how they will be based on both my experience with such things in the past (Si) and my opinions about what something means (a combination of both Fi and Ne).
I don't think I'm afraid of what's happening in the moment so much as I'm afraid of what the present will mean for the future. That is to say that I seem to generate a myriad of ways in which what is happening now could follow a chain of future events and lead to inevitable isolation, pain and disaster. When I'm scared of people, I get upset, internalize everything and sometimes if pushed too much, lash out and say things as they are without tact or remorse. Like an extreme version of Te, used irrationally and in self-defense.
I think that ISFPs probably do this as well, but perhaps they are more focused on how what is happening now will affect them. I would guess that, depending on the person, they would either take the thing that is bothering them head-on or they would simply run away from the perceived, real-or-not threat. They're probably more focused in on one thing, or a person that gives them a bad vibe, or a meal at a restaurant that might cause food-poisoning.
Any ISFPs who want to chime in are free to correct this, as that's just my impression of them rather than the perspective of one. They do have Ni in their function stack, after all, so I suppose they would become hyper-focused on details and read into things far beyond what they actually are. I don't know much more than that. I think there would be some kind of in-the-grip behaviour involved.