Despite the fact that Charlie Kaufman's script and Michel Gondry's visual concepts were closely followed, the actors were allowed many chances to improvise. Elijah Wood and Mark Ruffalo improvised extensively, and much of the dialog between Jim Carrey and Kate Winslet resulted from videotaped rehearsal sessions during which the two actors 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 shot and had to quickly sit down in the chair and visa-versa when he has to stand up.
Initially throughout the train scene, the music was supposed to fill up the gap during the silence between Joel and Clem until writer Charlie Kaufman suggested to do the opposite. Music was then played when Joel and Clem talked and paused when they paused.
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.
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 ever 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.
Michel Gondry had a unique system of controlling his camera operators while shooting by use of a headset for Michel 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 results 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.
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 1971 Hindi (Bollywood) movie "Gambler" 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 1987 movie "Dil Tujhko Diya", and is sung by Lata Mangeshkar.
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 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 likes it because Carrey's face appears so saddened. When the sound blanks out in the final film, Carrey is actually saying "Kate?"
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.
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 itself, and the voice of Katy Skjerping, an editor working at the production company Focus Features. Apparently, the young lady was asked to do a quick voice-over, before Winslet arrived, and it was kept in the film.
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."
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.
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. Chris Norr was a camera operator on the film. Linda R. Chen was an intern and a New York casting assistant.
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.
Virtually all of the most bizarre and fascinating scenes in this movie were created with old fashioned camera, editing, lighting and prop/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.
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.
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. Director Michel Gondry felt it was better to have Jim Carrey and Kate Winslet reacting to the children playing their characters as it happened.
Unnoticed visual effects were used in the movie, that were not planned while shooting. In one shot, Clem 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 Clem and Joel were breaking into, which collapses in a 4-second shot.
The trivia item below may give away important plot points.
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 very 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 sub-plot 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.