It is generally considered more appropriate to first read a book and then watch its adaptation, primarily because when you read the book first, the images of the main characters are formed independently in your mind. However, if you watch the adaptation first and then start reading the original source material, the cinematic images will involuntarily infiltrate your imagination. This means that you immerse yourself not in your own associations, but rather in associations 'imposed' by the director. Yes, these associations can be good, but they are unfortunately not your own and will never be.
With the film 'Solaris,' I had a similar experience. I first watched Tarkovsky's famous adaptation and then decided to delve into Stanislaw Lem's original novel. It seemed that reading the book would complement the complex film, providing a more complete picture of what happened on screen.
On one hand, the book indeed expanded on the theme of 'solaristics' more thoroughly, while on the other hand, clear differences between the film and the book emerged. Unlike the film, the action in the book takes place immediately during the approach to the Solaris station, without any earthly prologue. In the film, Kris hears about the possible visions on Solaris before he even reaches the planet, but in the book, Kris learns about such phenomena only from the documents discovered on the station, which is crucial to his perception. Meanwhile, in the film, Bannionis' character is already prepared for the journey and perceives the events on the station with less surprise.
The second significant difference lies in the emergence of the so-called 'guests.' In both the book and the film, they appear after sleep, but then the discrepancies arise. In the film, the 'guests' are essentially resurrected memories of the person. Yes, they materialize and cause suffering, reenacting unpleasant moments, but they are still memories. In the book, however, these 'guests' consist of two components: on one hand, they are undoubtedly memories (external appearance, moments from the biography, etc.), and on the other hand, these 'guests' reflect the thoughts of the person they visit. Harey, the main character's wife who took her own life in reality, is portrayed as a 'guest' in the book. She uses nicknames of spaceship pilots in her speech, which became known only after her death. So, her way of speaking combines additional information from Kris Kelvin himself. In the book, a 'guest' equals memories + personal thoughts. This, in my opinion, is an important clarification because, based solely on the film, it seemed to me that Kris was simply haunted by a memory transported into the present. It is much more challenging to rid oneself of a memory that speaks with your own thoughts and in your own language."
In general, I didn't see any contradictions between the book and the movie - both the book and the film are beautiful in their own way, in my opinion.
Of course, Stanislaw Lem was aware that there would be a film adaptation in the Soviet Union. Fortunately, there are documents that have preserved this information. In the book by Salynsky D.A. titled "Andrei Tarkovsky's Solaris: Materials and Documents," there is a message from the film's editor, L. Lazar, about Tarkovsky's conversation with Lem. The message is dated October 1969:
"Dear Tamara Georgievna! As per your request, I am informing you about the conversation with St. Lem. On October 19, Tarkovsky met with him to discuss further work on the Solaris screenplay. The remarks made by S. Lem on the screenplay concerned the part where the action takes place on Earth....<...>"
Thus, it is known for certain that Tarkovsky and Lem communicated during the creation of the film, and it is possible that some of Lem's wishes were taken into account by Tarkovsky. Although conflicts between writers and adapting directors sometimes occur, for example, one can recall the conflict between Stephen King and Stanley Kubrick regarding "The Shining" adaptation. Overall, the novel "Solaris" is quite interesting to read, and it doesn't feel dragged out. As a bonus for those who have read the book, there is a more detailed explanation of "Solaristics" and what is happening on the planet Solaris compared to the film.