Interesting about the Fi versus Fe observances. For me... I want my facial expression and body language to genuinely reflect what I am feeling. When I am forced to stifle these emotions it feels.... wrong.
I think it's the opposite for me. I'm supposed to be Inferior Fe, which is going to complicate things, but there are occasions where what I feel is so intense, sharing it with others creates problems. It's all or nothing.... there are no degrees. If I'm not sure that someone is ready for that, I don't see any good coming from me sharing it (for either myself, or anyone else). In my experience, it tends to either create problems, or intensify ones that already exist.
Whereas I notice my ENFJ friend is the queen of making comments like "and there I was having the worst day of my life but it was no excuse to not smile and look jolly."
I've found that if I'm unhappy, and I express that, I express that very strongly. This makes other people unhappy, because emotions are often contagious. But people don't like being unhappy. If there's nothing they can do about whatever is making me unhappy (which is often the case), why should I burden them? What's bothering me has nothing to do with them, so why should I put that on them? It also has a way of creating a loop of unhappiness, too, and prolonging the unpleasant feelings.
I think the most that can really be said is that, in general, maybe in theory Fe is more liable to use facial expressions as signifiers of commitment/social standing, while Fi is more liable to use expressions as signifiers of underlying emotional state... but that is just theory and assumes the extreme "idealized" function manifestation... a cliche instead of a human being.
Ah.... so this would be where the "Fe" is fake thing comes from. Fitting in with this, if I have emotions that people consider to be less positive, I prefer to hide them from people unless the emotions directly concern the people in question.
The reality is that no one is 100% one or the other, and so our facial expressions are not used in a uniform way on a practical, in-the-moment basis. If you're trying to use them to decode, it's like a cypher where the same letter can mean different overlapping things and an expression can stand for different letters.... from individual to individual. There is no uniform code.
Any human being is capable of expressing an authentic feeling on their face regardless of MBTI type; it's what human beings do. And all human beings are capable of smiling at others (or passing along some other facial cue) as part of a socialization strategy, regardless of MBTI type. It's all part of the human lexicon of human emotion and human community.
Trying to use a single photo / captured moment in time to decipher type from facial expression is about as useful as just flipping a coin.
Oh.... I'm in agreement here. I was interpreting this thread as being more about "tendencies" than hard and fast rules that can be applied in all situations. That's how I try to view this stuff... as a collection of "tendencies". I do think certain people are more likely to tend to think or do certain things than other people. I see MBTI or Jungian theory as an attempt to classify that. There's also the fact that it provides context for my oddness.