It's something to do with the forehead. And- harder to pin down- with the way he carries himself.
There are shots in Being There where it's definitely my father up there on screen.
Being There is one of my favourite movies. I watched it again last night. Sellers made a lot of cruddy films- many of them cruddier than they need have been because of his appalling behaviour on and off the set (Blake Edwards reckons he was certifiably insane)- but Being There is immaculate.