One of the reasons the film now enjoys cult status is because of its blackly comic last line. Ironically, Harlan Ellison hated this and actively campaigned to have it removed.
At a science-fiction convention, there was a pre-release screening of this film. At the end, when writer Harlan Ellison came onstage and invited comments, the most common one was that the portion of the movie taking place in The Underground was slow-moving (a polite way of saying "boring"). Ellison was very defensive about this and responded to the effect that The Underground was a boring, stultifying place and the movie was intended to convey that. The audience did not find this to be a satisfying answer.
James Cagney was considered for the voice of Blood, but the idea was dropped because Cagney's fame would have been a distraction from the character.
The extras in the outdoor movie theater sequence were soldiers from an army base.