In early versions of the script, the character of Evan was originally Chris Treborn. When the "T" is moved over, it becomes "Christ Reborn". This was changed to Evan Treborn, which is a play on "Event Reborn".
During one of Evan's "flashback" scenes, he can be heard reading part of Ray Bradbury s short story "A Sound of Thunder". In this story, a group of people travel millions of years into the past to hunt dinosaurs. One of them accidentally steps on and kills a butterfly, which dramatically alters the future.
Elden Henson shot all of the later "normal" scenes with his character first because he had to gain around twenty pounds in one month for the later timelines in which his character is crazy. He was to look bigger as crazy Lenny and thinner as normal Lenny. To the filmmaker's amazement he accomplished this.
Evan's diaries have the same cover layout as the diaries of John Doe in Se7en (1995), also released by New Line Cinema. They are standard composition notebooks that are used by school children across the country every day.
The scene where Evan has no arms was achieved by using two shots: one with an empty bed, and one with Ashton Kutcher lying in the bed with green gloves on his hands (which were erased later). Both shots had identical camera movements. For most films, this would be achieved by using machine-controlled cameras, which can replicate the exact same movement for multiple takes. However, since this film was fairly low-budget, the filmmakers were not able to afford this kind of equipment and the two identical shots were achieved by manually moving the camera while using a stopwatch for reference. Any small changes in the two shots were fixed digitally in post-production.
The flickering of the lights in the background of the psychiatrist's office when Evan is being hypnotized after they blew up the mailbox was not a planned special effect - it was an actual short in the wiring on set. The directors thought it fit well with the scene and used that take in the final cut of the movie.
During Evan's penultimate flashback (the one after which he wakes up in the mental hospital), there is a quick succession of shots with people making funny faces. One of the people appearing on several pictures is co-director Eric Bress.
The title is a reference a concept in Chaos Theory whereby small events may lead unpredictably to large events. This is explained in the opening quotation which explains that a butterfly flapping its wings may result in a typhoon. It may also be seen secondarily as a reference to the short story, "A Sound of Thunder", by Ray Bradbury.
The trivia items below may give away important plot points.
There are four alternate endings. In the first, Evan turns around and talks to Kayleigh at the same time. In the second, Evan turns around after Kayleigh has already turned around, and follows her. In the third Kayleigh turns around first and then when Evan turns around Kayleigh walks away. Evan stands there for a minute then walks away and does not follow Kayleigh. In the fourth one, Evan watches a home movie of his birth, rather than him meeting Kayleigh. He travels into the movie, and strangles himself in the womb.
The movie's title screen is a scan of the brain (as shown from the front). The left and right lateral ventricles of the corpus callosum (the middle of the brain) in this picture are displaced. Such is the case with many schizophrenic patients. The displaced ventricles in the middle of the brain are sometimes referred to as looking like a butterfly. In the movie, Evan is not schizophrenic, but in the next to last universe he "jumps" to (after he accidentally kills Kayleigh as a child), he is treated as if he is; the doctors tell him that everything that happened to him throughout the course of the movie was made-up.
Aside from appearing for a split second in a picture during the last-to-final flashback, co-director Eric Bress also appears as a patient in the mental hospital in the beginning of the movie. In the alternate ending on the director's cut, he also has a cameo as Andrea's new husband (in the alternate time line created by Evan).