Wouldn't this "expressiveness" be the common point of look-a-like types? INFJ and ISTJ are pretty much opposites in function, but their temperament is almost exact. The only difference is that one is phlegmatic, but that is still Cooperative.
Technically ISFJ and INFJ are reversible in temperament mel-phleg, phleg-mel. However phleg overtakes the ISFJs appearance where phleg of INFJ stands for idealism. So there is actually more temperament they have common that is in a different form rather than an alike form.
INFJ and ISTJ have expressiveness in common which stands for what they "say," but is it usually common because of this for them to act similarly (towards a specific situation, or in a general sense?) Situational reflects extroversion, which also reflects Sanguine and Choleric. Generality seems to reflect introversion, or Phlegmatic and Melancholic. If one is an idealist, then you would expect one to act phlegmatic, however since they are mainly melancholic, then much of this phlegmatic temperament is not seen because it is "responsiveness." So what is seen is that they are Melancholic and Cooperative. Is this not significant?
I guess you're right. I don't know many INFJ's in person, apparently (though there are a lot of people whose temperament from either system I wonder about, and haven't had the opportunity for them to be tested in either).
I do know one definite INFJ, (tested with the official MBTI at her job). On the surface, she looks like the other Melancholy women in our circle, who are ISTJ. Basically in bad moods and critical of people a lot of times. Yet there is something different that is not really that common among our friends (heavily SJ). She doesn't seem to be as
independent as the other Melancholies. So it's probably a different Control. She checks with others when making important decisions. My wife and I were saying, she's probably Melancholy-Supine. (She actually filled out the APS, but did not pay to have it processed yet, and now she's moved away). That would also fit in with NF, and the closest fitting Control range in FIRO is called "the Checker". She's also not as logically-oriented as the others. And I do get the same vibes I have gotten from Ni types, though I can't put my finger on why, or think of any positive uses of it from her (my wife says she's always predicting things about people, but it's always negative though. From what I've seen, Ni communication tends to have a natural appearance of "snarkiness" to me, which might be more of a shadow reaction on my part. Wilhemina Slater, the ENTJ villainness on Ugly Betty is the perennial example I think of).
So it seems that yes, on the surface, the INFJ is very similar to the ISTJ. Not very fast to take action (which would be the Cooperativeness shared with the SJ's), yet when you know the person, you see the need for more input from others (which would perhaps indirectly tie in with the NF's Motive focus). But you have to be able to get that close to them in the first place, to see that. The Interaction area would determine "Who is IN or OUT of the relationship" and then conation would determine "Who makes the DECISIONS for the relationship".
I would still think the ISTP would be closer to the INFJ, but then that type is pragmatic, and more physically oriented than even the SJ's, and thus more different on the surface; and you want strictly, surface behavior. So perhaps the Si and Ni types will look similar in that respect. So, OK, you've got a point there.
But it seems very few people in the theory of personality are concerned strictly with the surface. We generally want to go to the root of personality.