One of my closest friends is an INTJ male. On the outside, you'd probably think we were the same person...it is uncanny how we react to things in very similar ways.
Most of the time our thoughts are even in sync with one another (but that could just be b/c we've known each other for about 8 years). Of the differences I see between us, he is very intellectual at pretty much all times. He enjoys reading informational texts on all kinds of stuff, whereas I probably won't read about it for enjoyment...I'm more likely to just jump strait into a project, ask for help, and/or figure something out by actually doing it.
His thought processes are probably more fluid and changeable compared to mine...he expects things to go wrong, and will adjust accordingly. He's pretty good at improvising things -- me, not as much. Teaching has taught me to be flexible, but I still like being planned and rehearsed and decided on things ahead of time. He sees the "big picture" more easily, but I'm the one that catches the details.
Socially, I'm more outgoing and talkative, and can tolerate people a little better. He prefers to keep to himself, and shows very little emotion. He has 2 default facial expressions: smirk, and neutral. It is rare to see a wider range of emotion than that (could be a guy/girl thing, though?).