Carl Sagan Thought Kubrick Took His Advice on Aliens. Kubrick Had Cut Him Off Within the Hour.
The young Harvard scientist said showing aliens would ruin the magic, then took credit for their absence, while Kubrick kept trying to put them on screen.

Every Single Sci-Fi Film Ever* · 2001: A Space Odyssey (Part 2)Carl Sagan arrived as the expert in the room. The young astronomer was in his 20s, based at Harvard, and already trading letters with Arthur C. Clarke, the writer working with filmmaker Stanley Kubrick on 2001: A Space Odyssey. Sagan came to Kubrick's apartment to talk through the biggest question in the movie, how to show aliens.
According to author and filmmaker Michael Benson's account, Sagan's position was that showing the aliens would take the magic out of the film, and that Kubrick should never show them.
That, at least, is what Sagan later said his advice had been. Years after the movie came out with no aliens visible, only traces of their work, Sagan wrote with what Benson calls self-satisfaction, "I was glad to see that my advice was taken."
He had no idea what happened after he walked out the door.
Within an hour of Sagan and Clarke leaving the apartment, Kubrick telephoned Clarke at the Chelsea Hotel, where Clarke was staying and working. Benson notes that without mobile phones, a caller had to wait. According to Benson, Kubrick told Clarke, "Listen, get rid of him. I never wanna see him again. Do whatever it takes. Take him to the New York World's Fair. I want nothing more to do with him."
Benson says the clash was personal as much as creative. Kubrick was self educated, an autodidact who had learned by reading and doing, and he bristled at the manner of the young Harvard PhD who seemed sure he knew best.
The deeper irony is that the advice was not taken. Benson says Kubrick kept trying to find a way to show aliens and was drawn to the thin, spare forms of artist Alberto Giacometti. He never found a version that satisfied him, so the audience never sees them. Sagan took credit for an absence that, in Benson's telling, came from Kubrick's restless search, not from listening to him.
Questions people ask
So do we ever see aliens in 2001?
No. In 2001, aliens are never seen directly. Viewers see evidence of their intelligence, including the black monoliths and, near the end, a hotel room that seems built for a human's comfort, suggesting unseen makers who understand people.
What was the New York World's Fair Kubrick mentioned?
It was a huge international exposition in Queens, New York, in 1964 and 1965, with pavilions about science, space, and the future.
Was Carl Sagan famous at the time?
He became famous in the 1970s and 1980s as an astronomer, author, and television host, especially for the series Cosmos. At the time of the Kubrick meeting, he was a promising young researcher known mainly inside space science circles.