Occurs throughout, as Carrey and Winslet were encouraged to talk about their relationships and heartbreaks to each other and generate a real friendship to help their character chemistry.
When Stan scares Mary, Mark Ruffalo was instructed to hide in a different place each take to actually scare her.
Clementine punches Joel in the train scene. Kate Winslet actually hit Jim Carey, and his reaction is real.
The parade sequence is this in spades, as it was entirely coincidental that they caught it, so they decided to totally ad-lib things and film it. Not only are their reactions to the parade genuine, but Joel's response when Clem disappears is even Carrey's real reaction; he wasn't told Winslet was going to disappear, so Carrey's surprised, sad response really is him trying to find where she went.
Improv: 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 dialogue 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.
Misaimed Marketing: The Italian release had a misleading title - translating as "If you leave me, I deleted you" - and many Italians went into the movie expecting a comedy. The Spanish one, has "¡Olvídate de mí!" (Forget about me!), resulting in more people thinking it was a comedy.
From the supporting roles, Elijah Wood plays a shallow douchebag.
Practical Effects: 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.
The rising water in the beach house was created by building a house by the beach and waiting for the tide to come in.
Many of the "erased" book spines in the book store scene are just books turned around so that they are spine-in instead of spine-out.
Real Life Writes the Plot: While filming in downtown Manhattan, they heard that there was a circus parade going on outside. Gondry grabbed a portable camera, and got some footage of Carrey and Winslet messing around in the crowd completely anonymous due to it being night time and since the actors were wearing winter gear. The result is one of the happier memories in the movie.
Stan and Mary dancing over the unconscious Joel in their underwear was improvised.
The voice whispering "Montauk" is a combination of Kate Winslet's voice and an editor working at Focus Features. The woman was asked to do a quick voice over before Kate Winslet arrived, and they left it in.
Unintentional Period Piece: With the rise of social media since the film was made, it would be much harder to completely erase every indication of a person's existence from your life. This has actually become a big piece of Fanfic Fuel.
Viral Marketing: A website for Lacuna Inc. was created to promote the film. It contained a link to watch Joel experience the procedure, which would link to the movie's official website.
An early draft of the script began with an old woman — later revealed to be Mary — trying to publish a manuscript called Eternal Sunshine of the Spotless Mind, implied to be a tell - all about Lacuna. At the end of the screenplay, we discover that Mary is still working for Howard (who’s very, very old), and that Clementine and Joel have been erased each other from their memories and getting back together at least 15 times over the decades.
Joel makes several references to his ex, Naomi, the one he was living with when he met Clementine. He even considers getting back together with her. We never see her in the finished movie, but the character was in a few scenes in the original version (including a post-breakup hook-up). And her voice is heard over the phone in a deleted scene.
There was to be a montage of people who wanted their memories erased - a soldier seeing his dead friend on the battlefield and a girl who had been raped at a young age.
The original screenplay included a short conversation between Joel and Clementine 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 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.