Peeps really helped me out with my ISFJ questions here:
Pouncing Male ISFJ
My ISFJ and I had a conversation this weekend about how it is difficult for me to know where he is coming from because he is not so forthcoming regarding his feelings towards me. And he said, "Well, after three dates, I want to say that I am as interested as I can be without freaking anyone out... does that make sense?" It seems he is cautious not only for my sake, but for his as well.
Also, when referencing another girl, I asked why he had not made a move, and his excuse was that he was not sure if she was into him, and it seems that he needs to be pretty sure. He said he "put out feelers" and she did not seem responsive. This obviously was not very reassuring to me cause he never once said, "If she asked me out now, I wouldn't be interested." He only said, "I don't think she's interested." He was however immediately reassuring when I exclaimed that he could date her if he wanted to since it seemed like maybe we'd be better as friends (since he was not being direct with me). Still, he only offered a, "at this point, I don't
think I'd even be interested if she told me she was."
Works for me right now since I'm not really sure how I feel about him to begin with. But, I guess it is an example about how slow ISFJs may or may not be to express interest.
In my experience, I agree with Eiddy, in that "she is interested when you are the only person she seems to really want to talk to." And on the other hand, my ISFJ said to me that this is how he tells people are interested in him.