The Art of the Portraitist in the 19th Century

I look at some of those great 19th century portraits where everybody looks like some version of the American Gothic painting, absolutely rock solid expressionless and vaguely unhappy. And I keep thinking, what were they talking about during that session? How was this whole art form even explained to them or the intent of the photographer? Those must have been fascinating conversations back then. They probably didn't say anything because they were instructed not to move their mouths, not to blink. Studios had had clamps and things at the most extreme to keep people from blurring in the pictures because the exposures were so long.