The DU Lounge

In lieu of a villain, it's more common to see an "antagonist" who is just a character who forces the protagonist to face something and change as a result of encountering the antagonist. It can be a very subtle process. In some films, one could even say society is the antagonist ("Taxi Driver" is one example that comes to mind).

EDIT: Another interesting film to examine is "The Big Chill." That is a film with a group protagonist and they are changed by the death of a friend who the viewers barely see, and who ha no dialogue. We only are shown the body, briefly in the opening credits (IIRC).