For me, the best bits in vintage photographs are often the bits the photographer didn't really mean to capture.
The long exposures and bulky apperatus required by early photographic techniques meant that the candid shot was unlikely. People generally knew you were there, and they were paying attention. So you have the slightly disquieting sensation that everyone in the the street is staring at you.
And then there are the people along for the ride - in A.C. Barker's case often his kids. Here you can see two amongst the flax by the riverbank.
Those long exposures make representing people in movement tricky. But as most people are standing watching you anyway that wasn't too much of an insurmountable problem. Here my guess is that D.M. Mundy asked the fellow with the beard to stand still as if in mid stride - his frozen movement seems weirdly clunky.
But above all, these photographs can render perfectly normal people rather creepy. I know it's hardly breaking new ground as an observation, but... maybe it's all the beards. (And I say this as a committed beardy myself.)
The cab driver to the left of this Burton Brothers shot is almost definitely a serial killer...