A bit of both really. It's hard to describe because I tend to go immediately from seeing someone to forming an impression, but it's not a verbal one but an impression of character. It would take quite a bit of effort to rationalise and verbalise it for a particular person. It's not something clairvoyant or weird, but more like an instant character pattern recognition. I do the same with places too and express this in my photographs. I love doing it with places but dislike doing it with people, so I tend to avoid more than glance at folks unless I'm talking to them. If I'm out and about, I tend not to look at people very much - but then I'm usually in a world of my own anyway, and trust my feet to take me to where we're going lol.
I think some of the best portrait photographers and artists do what I'm describing with their subjects to very great effect - I wish I had that gift, but it's not something I'm comfortable with unless I know the person very well. I do take the occasional candid shot though - there's a delightful character fugue in these two people I photographed a few years back for example:
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I actually don't think what I'm describing is unique to INFJ types - I think a lot of other people do this too though they may not be as aware they are doing it as we and some of the other NF types are. The first impressions can be pretty spot on, but often they get refined and amplified deeply if we have real contact with someone. It may be that INFJs see more deeply than many others if they turn their attention fully onto a person, and soak up more input because Ni picks up all the subliminal information and structures it. On the other hand, it seems to me that if INFJs become emotionally invested in someone romantically, or as a close friend, their dreams get tangled with the reality of the other person, and we can be very off with our understanding of them then.