I live with an INTJ. The main differences I've noticed are in regards to his Te (extroverted thinking) and Fi (introverted feeling). INFJ's functions differing: Ti and Fe.
In regards to Te and Ti. He'll voice out his ideas, and develop through them out loud, arriving at his final conclusion not necessarily before he has spoken but while and after he has spoken/expressed his ideas. He'll often say: "I'm thinking out aloud here..." Whereas with Ti, I primarily do my thought processes in my head, usually only voicing ideas that I have already finalised. He'll also be more likely to comment on something (i.e. an article read out aloud by my ENFP friend), whereas I more frequently don't bother expressing what's in my head. Often when I do express something from within 'my head', I am only doing so for the sake of sociability (here comes Fe), since usually I don't even want to say anything and feel no need whatsoever - it's like: "Ah okay, I may as well say it."
Edit: I can't relate too much to the differences you've drawn between INTJ's and INFJ's, but I can see what you're getting at. In my experience INTJ's can be much more lighter and dynamic at certain moments than the INFJ, but this is only ever temporary, as the norm seems to be a sort of chilled-somber-plain-face which can be very hard to read! Fortunately vibes help one to gage their mood. But then again, INFJ's can have a similar plain-face. But Fe really is a noticeable difference to Fi. I live with three people with Fi!