* AdvertisedExtra: Creator/KirstenDunst is in way less of the film than the marketing campaign made it out to be.
* DeletedRole: Ellen Pompeo initially appeared in the film as Joel's ex girlfriend Naomi, who is only mentioned in the final cut. A subplot would have had Joel having a one night stand with her.
* DeletedScene: In the extended version of Mary's tape, we learn that [[spoiler:Mierzwiak pressured Mary into having an abortion as well as a memory wipe]].
* EnforcedMethodActing:
** When Stan scares Mary, Creator/MarkRuffalo 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.
* FakeAmerican: Kate Winslet and Tom Wilkinson are both British actors playing New Yorkers, as well as Jim Carrey, who is Canadian.
* {{Improv}}: Despite the fact that Creator/CharlieKaufman's script and Creator/MichelGondry's visual concepts were closely followed, the actors were allowed many chances to improvise. Creator/ElijahWood and Creator/MarkRuffalo improvised extensively, and much of the dialogue between Creator/JimCarrey and Creator/KateWinslet resulted from videotaped rehearsal sessions, during which the two actors became close by sharing tales of their real-life relationships and heartbreaks.
* MisaimedMarketing: 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.
* PlayingAgainstType:
** As opposed to their usual roles, Jim Carrey plays TheComicallySerious while Kate Winslet plays the ManicPixieDreamGirl.
** From the supporting roles, Creator/ElijahWood plays a shallow douchebag.
* PracticalEffects: 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 ForcedPerspective 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.
* RealLifeWritesThePlot: 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.
* ThrowItIn:
** 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.
* ViralMarketing: 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.
* WhatCouldHaveBeen:
** 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 has had Joel erased from her memory 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.
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