It's very difficult to hold a smile very, very precisely. Any movement would've meant the colors would smudge. Not necessarily the reason why they're not smiling, but it's a possibility, I believe.
That's exactly it. The exposures were much longer then than now. It's also the same reason the bodies of water have the rainbow effect on their surfaces.
That chromatic effect is specifically the result of different intensities of light tracing different paths across a highly variable reflective surface (water) to the slightly different locations of each of the three separately filtered lenses of Prokudin-Gorskii's apparatus. It's an effect of space, not time.
Agreed, but holding any pose is a challenge - kids sitting on the hill - they'd fidget and move. The water moves. The sun moves if you wait long enough. I wonder how many 'bad' photos there are from the early days of photos where people tried smiling and they had smudged faces.
