Initially, throughout the train scene, the music was supposed to fill up the gap during the silence between Joel and Clementine, until Screenwriter Charlie Kaufman suggested to do the opposite. Music was then played when Joel and Clementine talked, and paused when they paused.
Despite the fact that Charlie Kaufman's script and Michel Gondry's visual concepts were closely followed, the cast members were allowed many chances to improvise. Elijah Wood and Mark Ruffalo improvised extensively, and much of the dialogue between Jim Carrey and Kate Winslet resulted from videotaped rehearsal sessions, during which, the two became close by sharing tales of their real-life relationships and heartbreaks.
When Joel is in his head, and is visiting his session of the erasing process, no special effects were used to show the two Joels in the one scene. Jim Carrey had to take off his hat and jacket when he was not in the shot and had to quickly sit down in the chair, and vice-versa when he has to stand up.
The scene where Joel and Clementine watch the circus go through the streets was made up on the spot, as the film crew and cast happened to be working nearby, and Director Michel Gondry decided it could work well in the film. The part where Clementine disappears suddenly, is one of Gondry's favorite moments of the film, as Jim Carrey didn't know Kate Winslet was going to disappear, and Gondry liked it because Carrey's face appears so saddened. When the sound blanks out in the final film, Carrey is actually saying "Kate?"
Clementine's hair goes through several color changes, blue, orange, red, green, and brown, which seems to be her natural hair color. This helps the viewer keep track of where her relationship with Joel corresponds to the plot.
Virtually all of the most bizarre and fascinating scenes in this movie were created with old fashioned camera, editing, lighting, and prop and set tricks. The use of digital effects was very limited. The striking kitchen scene with Joel as a child, was created with an elaborate forced perspective set-up similar to some used by Peter Jackson in The Lord of the Rings trilogy.
Michel Gondry had a unique system of controlling his camera operators while shooting, by use of a headset for himself and earpieces for the two operators. He would speak to them (in French) while cameras were rolling and the actors were doing their parts, so Gondry could have a say on all angles no matter where the actors were. This resulted in a large degree of spontaneity, since the actors could decide while in character whether to have an entire conversation sitting on a couch or get up and walk to a window. Kate Winslet said that she felt this freedom enhanced her performance, and that sometimes they would do different takes of the same scene completely differently, based purely on gut feelings for what the characters might have done.
The movie is based on the following quote from an Alexander Pope poem, "How happy is the blameless vestal's lot! The world forgetting, by the world forgot. Eternal sunshine of the spotless mind! Each pray'r accepted, and each wish resign'd."
The audio for the scene in which Joel and Clementine appear as children in Joel's memory, while their adult voices converse, was recorded on-location rather than dubbed later in a studio. Michel Gondry felt it was better to have Jim Carrey and Kate Winslet reacting to the children playing their characters as it happened.
Reporters tried to interview Jim Carrey as the unplanned scene with Joel and Clementine at the street parade was being filmed. If you listen closely, you can hear somebody shout "Speak to me!" at Jim Carrey.
Kate Winslet's different hair colors were achieved through wigs, not dyeing. Since the film wasn't shot in sequence, she sometimes had to have different colors on the same day, so dyeing wasn't practical. Reportedly, the red one was her favorite.
Using Clementine's hair color, viewers can keep track of what phase of the relationship they are watching: green for first meeting, red for fantasy scenes, orange for full-on affair, and blue for their post-cleansing re-acquaintance.
To help promote the movie, a fully functional website was created for Lacuna (http://www.lacunainc.com) purporting to provide memory erasure. The only giveaway is the link to watch Joel Barish "experience the procedure," which links to the movie's official site.
The voice whispering "Montauk" in the movie is actually a combination of Kate Winslet's voice echoing and the voice of Katherine Skjerping, an editor working at the Focus Features production company. Apparently, Skjerping was asked to do a quick voice-over before Winslet arrived, and it was kept in the film.
The memory-erasing company, Lacuna, Inc., takes its name from the Latin word meaning a cavity, hollow, or dip, especially a pool or pond. Transfiguratively, lacuna comes to mean a gap, deficiency, or loss. The term "lacunar infarct" refers to a stroke that involves a small area of the brain responsible for a specific function, or even a specific memory. Additionally, in papyrology (the study of ancient manuscripts), a lacuna is a hole where part of the text is missing, and which can sometimes be re-constructed.
Unnoticed visual effects that were not planned while shooting were used in the movie. In one shot, Clementine is walking on the street while a car falls in the background. The whole background was replaced with a CG-background, including Clementine's other leg which disappeared, so the remaining leg was done with CGI. Another shot done in CGI was the house Clementine and Joel were breaking into, which collapses in a four-second shot.
The original screenplay by Charlie Kaufman included a short conversation between Joel (Jim Carrey) and Clementine (Kate Winslet) about the album "Rain Dogs" by Tom Waits during one of the opening scenes on the train. During this conversation, Joel says he remembers buying the album and liking it, but he can't remember anything about it. While the dialogue was stripped from the film, during the fast shots of Stan (Mark Ruffalo) showing Joel the items he has brought in that remind him of Clementine a copy of the CD "Rain Dogs" can be seen for just a moment. Also the "blue ruin" reference comes from a lyric on the same album.
Mary's surname does not appear in the credits, but her nameplate on the reception desk at Dr. Mierzwiak's practice shows it as Svevo. Stan also says her full name an hour and a half into the film. This very unusual name is clearly a reference to Italian writer Italo Svevo (real name Ettore Schmitz, 1861-1928), who was very interested in the work of Sigmund Freud, and is believed to have corresponded with him.
The small computer hooked up to the memory erasing headset is an old Amstrad PPC 512 or 640, manufactured back in 1988. Fittingly, the Amstrad PPC (Portable Personal Computer) had a reputation for having limited memory.
A lacuna is a "lake" seen on medical imaging as a hole filled with fluid within the brain, after some strokes and seizures. Such tiny "holes" can result in symptoms such as memory, sensory, and motor dysfunction, and are perhaps a reference to the "brain damage" that results from the procedure in the film.
When Joel goes to Lacuna, Inc. for the first time, he looks at cards being printed out with the names Chris Norr and Linda Chen. Christopher Norr was a Camera Operator on the film. Linda R. Chen was an intern, and a New York City Casting Assistant.
In the scene where Clementine invites Joel to her apartment for a drink, one of the songs playing in the background on Clementine's stereo is from the Hindi (Bollywood) movie Gambler (1971) and is sung by Mohammad Rafi. The song after that, which plays immediately after Clementine says "I'm gonna marry you", is 'Wada Na Tod' meaning "Don't break your promise" in Hindi from the movie Dil Tujhko Diya (1987), and is sung by Lata Mangeshkar.
In the DVD extra "A look inside Eternal Sunshine of the Spotless Mind", Kate Winslet is on-set at a beach talking to the camera next to Michel Gondry. He is wearing headphones and possibly cannot hear what she is saying. She says, "I'm telling my director that he's irreplaceable. What would I do without my little frog?"
Kate Winslet is the only star from the main cast who has not played a character in a comic book film; Jim Carrey played the title role in The Mask and the Riddler in Batman Forever, Kristen Dunst was Mary Jane in Spider-Man and its sequels, Mark Ruffalo has been The Hulk since Avengers, Elijah Wood was Kevin in Sin City, and Tom Wilkinson was Carmine Falcone in Batman Begins.
Additionally, Deirdre O'Connell, who plays Dr. Mierzwiak's wife Hollis in this movie, played Anna Nelson in the third season of Daredevil (2015).
The original script featured a cut beginning and ending sequence that took place in the future. In the end, an older Clementine comes in to have the procedure done, and a look at her screen shows that she's had the procedure done multiple times, and all of them involved Joel. At the end of the script, an older Joel calls Clementine to ask why she hasn't called, but the technicians performing the procedure erase his message. Other cuts in the original script, include a montage of memories people wanted erased, including a soldier seeing his dead friend on a battlefield, and a girl who was raped at a young age. Another subplot dropped from the script, was Mary (Kirsten Dunst) finding out that Howard (Tom Wilkinson) made her get an abortion after they had the affair, resulting in her desire to have her memory wiped.
At least one version of the screenplay ends with Clementine and Joel in an endless cycle of erasures and getting back together. In the last scene, an elderly Clementine visits Dr. Mierzwiak, and the screenplay calls for the audience to see a computer screen showing "a list of fifteen dates of previous erasures stretching back fifty years, all of them involving Joel Barish." The actual ending used in the movie ends on a much more upbeat note instead.
On the album "Origin" by the band Dayseeker, there is a song called, "Spotless Mind". The song and lyrics is a summarization of the plot and scenes through Joel's perspective. Some lines from the movie are used as lyrics, such as,"I want to call it off", and "Please let me keep this one." The song ends with, "I'll find you when I wake and we'll try again, I'll meet you in Montauk my dear old friend."
An early clue that the start of the film is the end of the story, is the fact Joel says in narration that it's Valentines Day 2004 when he skips work toward the start, but in one of his memories with Clementine, he says it's November 2003.