Twenty years ago this week, David Fincher’s breakthrough film Seven slithered its way into theaters, and into the consciousness of an unsuspecting public. But its ingénue’s head in a box wasn’t the only surprise the grisly picture had in store (oh, um, spoiler warning? With more to follow?); the third-act appearance of Kevin Spacey as killer John Doe blindsided opening weekend audiences, since Spacey was unbilled on posters and in the opening credits, and unseen in the trailers and TV spots. It’s a good trick — and part of a long-running tradition of surprise, unbilled appearances by actors and actresses of note.