According to Douglas Trumbull, the total footage shot was some 200 times the final length of the film.
The initial idea for the device that would eventually become the black monolith involved a transparent screen, which would show the australopithecines how to use objects as tools and weapons. Sir Arthur C. Clarke later dismissed it as 'too naive'. Also, the H.A.L. 9000 computer started out as a mobile robot, but as Clarke feared that this view of artificial intelligence would become hopelessly out-dated in the coming decades, the omnipresent red eye was conceived.
The movie was not a financial success at first. MGM was planning to pull it back from theaters, but several theater owners persuaded them to keep showing the film. Many owners noticed increasing numbers of young adults attending the film. They were especially enthusiastic about watching the "Star Gate" sequence under the influence of psychedelic drugs. This helped the film to become a financial success.
At the premiere screening, 241 people walked out of the theater, including Rock Hudson, who said, "Will someone tell me what the hell this is about?" Sir Arthur C. Clarke once said, "If you understand '2001' completely, we failed. We wanted to raise far more questions than we answered." Clarke later expressed some concern that the film was too hard to follow, and explained things more fully in the novelization and subsequent sequels.
Stanley Kubrick worked for several months with effects technicians to come up with a convincing effect for the floating pen in the shuttle sequence. After trying many different techniques, without success, Kubrick decided to simply use a pen that was adhered (using newly invented double-sided tape) to a sheet of glass and suspended in front of the camera. In fact, the shuttle attendant can be seen to "pull" the pen off the glass when she takes hold of it.
Stanley Kubrick: [Bathroom] David Bowman explores the bathroom next to his celestial bedroom after travelling through the Star Gate.