The interesting thing about 2001, as a movie and as a book, is it was a mutual project across two media by two artists who rarely worked with others. After Clockwork Orange two years later, Kubrick never did sci fi again (although he kept working on AI as long or longer than he worked on his unmade Napoleon piece) and Arthur C Clarke never wrote another screenplay. In the opening of 2010, Clarke explains that the second book is a sequel to the movie, not the book, for the simple reason that the movie had a greater audience and the divergence was so great that a choice had to be made.

What makes it more interesting is it was a project where the weak spots of both artists weren't cancelled out, they were magnified.

Arthur C Clarke, as flagamuffin mentions, sucks at people. He always has. I've often described him as a Conde Nast Traveler writer going on an imaginary vacation; his descriptions of places and things are evocative and colorful but the people he places in them are cardboard cutouts. Things happen to people in Clarke stories, the people never make things happen. And, lo and behold, everything happens to people in 2001. With the one notable exception where Bowman manages to not die, every other event in 2001 is "something happens to people." More than that, the internal motivations and emotions of characters in Clarke stories are effectively absent. On screen, 2001 unfolds as a bunch of emotionless ciphers moving about the board like a bunch of non-player characters.

Kubrick, for his part, packs a lot of emotional depth and interest into tense situations. But aside from the monkeys, 2001 is about people who must have tight emotional control as part of their job descriptions. The end result is certainly tense but it's noteworthy far more for its imagery than its acting.

Kubrick was also brilliant for two acts. I can't think of a Kubrick movie that doesn't devolve into nihilistic denouement in the third act. 2001 is particularly guilty as the entire third act is effectively Douglass Trumbull masturbating on the slitscan followed by an incomprehensible and slow adventure in the world's cleanest hotel.

Here's the thing:

The ending of the book is clear. Bowman goes through the stargate, evolves into the starchild, returns to Earth and saves the planet from USA-USSR nuclear annihilation. It's explicit and obvious. The ending of the movie is not. They had no fucking idea how to film it. There was the constant assumption that they'd work it out, they'd fix it in post, they'd get the idea across and it wouldn't suck. There would be omnipotent aliens, there would be transformation, the world would be saved.

Eventually, they figured out that Carl Sagan had been right years earlier:

They had no idea how to end the movie - that's when they called me in to try to resolve a dispute. The key issue was how to portray extraterrestrials that would surely be encountered at the end when they go through the Star Gate. Kubrick was arguing that the extraterrestrials would look like humans with some slight differences, maybe à la Mr. Spock (Ed. note: like Clindar). And Arthur was arguing, quite properly on general evolutionary grounds, that they would look nothing like us. So I tried to adjudicate as they asked.

I said it would be a disaster to portray the extraterrestrials. What ought to be done is to suggest them. I argued that the number of individually unlikely events in the evolutionary history of man was so great that nothing like us is ever likely to evolve anywhere else in the universe. I suggested that any explicit representation of an advanced extraterrestrial being was bound to have at least an element of falseness about it and that the best solution would be to suggest rather than explicitly to display the extraterrestrials.

What struck me most is that they were in production (some of the special effects, at least) and still had no idea how the movie would end.

Kubrick's preference had one distinct advantage, an economic one: He could call up Central Casting and ask for twenty extraterrestrials. With a little makeup, he would have his problem solved. The alternative portrayal of extraterrestrials, whatever it was, was bound to be expensive.

So Kubrick ended up not showing aliens in a script that had always had aliens, for a movie where the actors have been unallowed to convey emotion in a story where they do exactly nothing to move the plot along.

And I love it. It's an incredible work. Nobody has touched the verite of 2001 in fifty.fucking.years and the ending is a star-spangled clusterfuck.

It's interesting to me because in 2007 I got to pitch the Sci Fi Channel on an alien contact movie. We'd come up with the idea the day before the call and my buddy observed that the problem with any monster/alien movie is the minute you see the monster you cease to believe. So I wrote an alien contact movie without aliens... and then read up on the Sci Fi Channel and opted not to pitch. But I wrote the script anyway. It remains one of the two works I've ever optioned.

Don't show the monster. But hey, if you can, also don't plan to show the monster. That way your movie makes sense even without dropping acid.