The flip side of what you are asking, i.e. how to get to know someone well enough in advance to try to gain entry into their emotional inner sanctum, that approach might well blur the line between objectivity and subjectivity. It will depend on your subject and your intentions, which could be journalistic or glamour.... or you might have some other more nuanced intention somewhere within the vast artistic expanse in between.
Some photographers simply seem to have more innate ability to read people's expressions and know when the subject is in their element and expressing themselves in a way that makes a compelling photograph.
As for me, I tend to shy away from most direct forms of portraiture; I suppose it's because I find it quite invasive, and really requiring an emotional connection. Being a scientist at heart I tend to cling to the (silly?) notion that I am merely observing a 'found' subject and not changing it or influencing it in any way. Anyway we all have something to work on!
Just a piece of trivia, I recall when reading about early photography of the native Americans, it was quite problematic to take their photographs because many of them considered the eyes to, literally, be the doorway to the soul and thus found the photographs very invasive and 'soul capturing' or perhaps even stealing. Indeed one does not see smiling native Americans in any of those early photographs. Anyway, just an interesting aside, and what I read might well have been coloured by some cultural bias.