Renowned astronomer Seth Shostak was hired as a consultant on the film. He reviewed the script several times for errors, and gave suggestions for making the scientists less dry: "Real scientists don't describe an object entering the solar system as 'notable for the fact that it was not moving in an asteroidal ellipse, but moving at nearly 3*10 to the 7 meters per second'. More likely, they would say that there was 'a god-damned rock headed our way!'" He also noted the scientists should refer to one another by a first name basis.
According to the filmmakers, John Cleese was the most difficult choice in casting, as he was primarily noted for comic roles. Cleese felt that at his age, a dramatic role with subtle humor would be an easier role to play, rather than another manic old man.
In the original movie, GORT was eight feet tall. In this film, he is 28 feet tall.
Keanu Reeves recorded the line "Klaatu barada nikto" twice, and one recording was played backward and spliced with the other (which was left normal) to make the overall dialogue sound more otherworldly.