Referring to the word 'but' in your sentence, being N has nothing to do with being unresponsive to body language.
INTJs are given the stereotypical description of being blind to body language. From what I have noticed this is incorrect. They are quite good at identifying and interpreting body language. They might not be very reactive to them.
ExTJs suck at being responsive and interpreting them though. That should be noted. Especially ENTJs. And IxTPs also might suffer from the lack of being responsive to them. INTPs the most.
Anyway I love it. Love getting better and better at it. The older I get the better I seem to get at it.
unresponsive? i dont believe i ever implicated anything which could give out such an idea. although i have noticed, N's are generally lousier in body language, or body language shy.
while S types never seem to give quite the depth an N who loves body language can, but then, this is N talking and thus N perspective and = biased as both look for different things from the same object.
such are the reasons, in maturing we find. (to your post)
to the fake smile thing:
i have an easier method, simply read their reaction. its easy to see what caused a reaction when youve learned to predict others.
the spotting some specific thing out of ordinary is T kind of observing just some very specific thing instead of depth or accordance.
Most of being successful at reading body language is simply paying attention to it.
with my love to it, ive analyzed it in depth to the point i know exactly when to look at others to know what i want to know and at other times, just absentmindedly eyes picking it while i concentrate on talking.
the more i concentrate on talking, the more blind i become. its funny how i sometimes when in a very successful conversation, realize im so in the feelings and conversation, that im really unconscious as to whats happening in front of my eyes.
(if i recall these memories, when i actually in the conversation realized im blind, and tried to see, that moment i remember as seeing through a tunnel vision, very blurred!)
well, i suppose its N's advantage when learning the s ways. you can simply analyze how it works, and then you know. (without having to concentrate on it thus distracting your brain processing power from what you could use to talking.)