Thankfully, the variety of posed O-scale figures from several providers is sufficient for scenes on our layouts.
I previously created a mountain scene for a modular layout and found wild animals for it -- deer, a family of black bears, and a group of cougars. I looked for a group of rock climbers for placement on a craggy cliff, but didn't find any. I think that means the "hobby" of rock climbing hasn't yet captured the attention of model figure-makers for production.
I recently installed RMT seated passengers in the 10 coaches of my MTH Aerotrain. The size was just right for that RailKing rolling stock. I initially tried to fit Lionel seated passenger figures in the coaches, but they were slightly too tall -- IMHO, better suited for 1:48 passenger cars.
Mike Mottler LCCA 12394