Solaris is the third adaptation of Polish author Stanislaw Lem's novel of the same name, the first two having been a Russian TV movie and a Russian feature film, the latter of which was directed by Andrei Tarkovsky. Like the other two adaptations, Solaris maintains the basic premise, plot and character types of the novel but diverges from the source in many respects, in particular its themes and how the characters are presented. This comparison will examine only this film and the original novel, and not the two previous adaptations.
Both the novel and film focus on a psychologist named Chris Kelvin (Kris Kelvin in the novel), who lost his wife Rheya to suicide a few years before the story begins. Both the novel and film depict Rheya killing herself after Kelvin leaves her, though the film elaborates by depicting Rheya as bipolar and having an abortion in fear she would pass her illness onto her child. Kelvin leaves in a rage after finding this out. Kelvin is summoned to a space station orbiting the distant planet Solaris in a message from Gibarian, who in the novel is Kelvin's old instructor but in the film is shown to be a colleague and friend. Upon arriving at the station, Kelvin discovers that Gibarian has killed himself in the intermittent period.
The Solarist scientists have been visited by entities created by the planet below, these "Visitors" being constructed from each scientists' memory and replicating a person they have known in their life. Gibarian's Visitor in the novel is an unidentified, Amazonian black woman, wherein the film it's a replica of his young son. The two surviving scientists have Visitors of their own, though both the scientists and their Visitors differ between the novel and film. While both versions of Snow are receptive towards Kelvin, the novel depicts him as an older, possibly alcoholic scientist, and the film portrays him as young and eccentric. Snow's Visitor in the novel is never described, but in the film "Snow" is revealed to be a Solarist Visitor himself, presumably taking the form of the real Snow or his brother, who kills the scientist in self-defence and assumes his identity an unknown interval of time before Kelvin's arrival. The novel's other scientist is Sartorius, a reclusive and analytical individual whose Visitor takes the form of a child or dwarf. In the film, the other scientist is Gordon, a similarly reclusive female scientist who holds a fearful and antagonist relationship with the Visitors and whose own Visitor is never revealed.
The planet Solaris differs vastly between the film and its source material. While the film portrays it as a gaseous-electrical planet with hints of sentience, the novel delves deeply into the planet's construction and history, describing it as oceanic and capable of making massive, highly symmetrical structures beyond human understanding and seeming to be actively conscious.
Finally, the novel and movie explore different themes. With the book, Lem attempts to craft a truly alien culture, with the planet Solaris seeming to be a massive, sentient organism in its own right with thought processes and modes of communication and expression that perpetually boggle human researchers; as such, the novel focuses on the impossibility of contact with a truly alien culture. The film uses the Visitors to explore relationships, namely Kelvin's attempt to understand his wife's suicide and even attempt to make changes, though he is unsuccessful in this regard, as this "Rheya" is a copy of his memories of his deceased wife and not actually her. While the novel ends with Kelvin unsure of how to pursue his future, the film has him opt not to return to Earth and instead let the planet consume him while on the station. Kelvin ends up in a sort of afterlife modeled on his apartment, where he is finally reunited with Rheya.