Regarding the Apollo 11 post-flight press conference, my own simple explanation is that at the start of it, all three astronauts probably felt many, many times more uncomfortable about being there than they felt a few weeks before about going to the moon and landing on it.
Excellent post, Peter B. As usual!
I have watched many of the HB videos concerning the press conference and attempting to describe body language and facial characteristics of the three during the question/answer period.
Their descriptions usually match exactly what the HBs want them to match, and they are so sure that they are right.
My own impressions of body language and facial expressions after more than 30 years of dealing with the public, are that they can be useful at times in face-to-face situations when there are other signals present, but there are plenty of exceptions to the "rules." Nervousness or confusion or discomfort can override everything. And some mannerisms have no meaning at all -- they are just trivial actions or even bad habits, and nothing more.
Much of the time, I prefer to not continuously look people in the eye when they are talking to me, and also sometimes even when I'm talking, which I'm aware could give the impression that I'm dodgy or dishonest. But doing so often gives me a sort of emotional overload which blocks out their words, so I'd rather look away and concentrate on just their words -- take them literally rather than make up airy-fairy nonsense about what they "really mean."
However, if I'm dubious about what someone is saying I'll watch them for clues, and will look people in the eye when I'm trying to convince them I'm being honest. Which I am most of the time anyway.
I've even come across badly-trained salespeople who overdid it by getting out their catalogue and, instead of pointing at something in it with one finger, pointed with their full, open hand with their palm facing me. It was so corny, insincere and laughable!
On one occasion I had a rare few minutes to toy with a young, new salesman who was over-enthusiastic about some new product that underwhelmed me, so I first tried to convey my disinterest verbally and got no response.
He was only part-way through his text-book spiel. So I said I had little time left and slowly moved back from the counter, leaned back against the wall, turned my body slightly away from him, folded both arms tightly across my chest, hunched my shoulders forward in a shrug, crossed my ankles, locked my knees together, and gazed up at a far corner of the ceiling while he still prattled on... Until I abruptly excused myself with "Gotta go," darted back to the task at hand and hoped he learnt something. Assuming that he noticed anything!