There could be other characters who interact with the main ones in different stories. Maybe a friend or a stranger who comes into the life of the main character and changes their perspective. For instance, someone who challenges the main character's view of an event and makes them see things differently.
I'm not entirely sure without reading the book more closely, but likely there are characters like the photographer in 'blow up'. He seems to be a central figure in that particular story, his actions and observations driving the narrative forward.