I can distance myself from what was placed up on the screen. I can separate the film from my reality. The documentary would probably elicit a different reaction. I think my fear would be more explicitly about leaving the theater. I would see myself in the faces of the documented people.
Truth is a very powerful metaphor.
It removes the abstraction, placing you not in a fictional world that may or may not be related to your own, but in a world whose power to affect comes from the knowledge that what is on screen is reality. V. Fiction Tom Stoppard once commented that if one sees an injustice occurring outside one's window, the least useful thing one could do is write a play about it. When I first heard his comments, they struck me as absurd. But, the aesthetic distance problem plagues such fiction. A novel is not an expose.