George Clooney said that Steven Soderbergh was originally considering another actor for the lead so Clooney wrote him a letter asking to be considered as he thought a letter would be "less personal" if Soderbergh wanted to turn him down. Clooney said, "thank God the other actor turned it down." The other actor was Daniel Day-Lewis.
Soderbergh cast Jeremy Davies in the film after seeing a tape Davies made of himself playing Charles Manson. Davies was preparing to play Manson in an independent film. The tape was passed around within the film industry and became a popular bootleg. Davies eventually played Manson in Helter Skelter (2004).
Was originally given an R rating by the MPAA primarily due to a pair of shots of George Clooney's nude rear end. Steven Soderbergh appealed the decision, citing that similar content (and worse) had appeared on network television. Soderbergh won the appeal and the movie was granted a PG-13 rating.
In the final train scene, a Chicago L platform sign (Merchandise Mart) is clearly visible, though out of focus. Footage of the sign must have been looped to make the station seem much longer than it actually is: on the side of the station that's open to backlight, there are only two such signs mounted on metal balustrades, about 25 feet apart, but in the movie the train appears to pass more than a dozen.
There is a sign on Athena's engines stating "Dals Ed Mitt i naturen" Dals Ed is a small town in western Sweden close to the Norwegian border. The artist put it there because "Athena must have passed Dalsland sometime".
Two character's names vary a great deal in the different versions of this story. The female character is known as "Rhea" in the English translation of the novel, "Rheya" in the film, and "Harey" in the original Polish. In the 1972 version, she is known as "Hari/Khari" in the subtitles and Russian dialogue, and "Carrie" in the English dubbing. The Doctor is known as "Snow" in the English translation of the novel, and the 2002 film, but in the Tarkovsky version, he is known "Snaut" in the subtitles and Russian dialogue, and "Stroud/Strowd" in the English dub.
In Lem's novel, character Rheya is named Harey, an anagram of Rheya. This is probably a reference to the titan "Rhea", who was mother of the Gods in Greek mythology, and which is also the name of a moon of Saturn. Rhea was the wife of Cronus/Saturn (the ruler of time), and the sister of Oceanus (Solaris is an ocean planet) and Tethys, a sea goddess, as well as the mother of Poseidon/Neptune; her parents were Uranus (the sky god) and Gaia (the earth goddess).
Kelvin shares his name with William Thompson, First Baron Kelvin, the legendary physicist who first determined an accurate value for absolute zero temperature. The corresponding temperature scale is named in his honor.
The DVD Special Features include a draft of the Screenplay in which Rheya reveals to Kelvin that she doesn't feel she can be a mother early on in their relationship, the Solaris Crew give Rheya's Phi-Creature a brain-scan showing that she has been reproduced down to the details that cause her medical condition, and in which Kelvin asks Snow if Solaris is God and Snow answers that Rheya would have been more likely to know.
The trivia item below may give away important plot points.
On Stanislaw Lem's personal website, shortly before he passed away, he posted that he felt this adaptation had overemphasized the human relationship dilemma in his novel, when he had been trying to propound what an Encounter with something truly Alien might be like. The planet Solaris is about as far from any preconceptions of Alien life as you can get, so identifying with Rhea's Phi-Creature misses the point that all the Phi-Creatures are Solaris.