* ActorInspiredElement:
** Jim was written to be British but Creator/CillianMurphy struggled to do an accent - so they let him use his natural voice. London has a high number of Irish immigrants anyway.
** Creator/NaomieHarris worked with Creator/DannyBoyle to come up with a backstory for Selena's cynical attitude; they decided that Selena had to kill her whole family in one afternoon. Knowing this explains why she is so heartbroken for Hannah after [[spoiler: Frank is infected]] and her speech to Jim about their family. It also adds another layer onto her being so protective of Hannah in the latter part of the movie.
* CareerResurrection: After the triple failures of ''Film/ALifeLessOrdinary'', ''Film/TheBeach'' and a straight-to-TV AwfulBritishSexComedy called ''Vacuuming Completely Nude in Paradise'', many people assumed that Creator/DannyBoyle would be another [[CreatorKiller flash-in-the-pan]] [[TheNineties 90s]] Indie director. [[Film/{{Millions}} Oh,]] [[Film/{{Sunshine}} how]] [[Film/SlumdogMillionaire they]] [[Film/OneHundredAndTwentySevenHours were]] [[Film/SteveJobs wrong]].
* CastTheExpert: Sort of. Only extras who had athletic backgrounds were cast as the infected, because Danny Boyle wanted to convey that they were significantly stronger than the protagonists.
* DarkhorseCasting: Creator/DannyBoyle intentionally sought out unknown and obscure actors for the majority of the leading roles, as he wanted the film to be discovered and not rely solely on star power. Creator/CillianMurphy, Creator/NaomieHarris, Noah Huntley and Megan Burns were virtually unknown prior to the film production, with Murphy and Huntley having only had minor roles in films, Harris only having done British television, and Burns having only had one previous acting credit. The only famous actors of note that were cast were Creator/BrendanGleeson and Creator/ChristopherEccleston. This also added to the tension of the plot, since viewers ''couldn't'' rely on star power to predict who's going to make it to another scene.
* DyeingForYourArt: Creator/ChristopherEccleston and the rest of the actors playing the soldiers did a three day boot camp to prepare.
* FakeBrit:
** Frank is played by Irish actor Creator/BrendanGleeson.
** Scouser Megan Burns attempts a London accent for Hannah.
** Cillian Murphy initially used an English accent for Jim, but struggled, and so Jim was made Irish instead.
* FakeIrish: Emma Hitching, who plays Jim's presumably Irish mother, is British.
* FocusGroupEnding: The original ending had Selena and Hannah failing to save Jim in a hospital and then ended with the two women walking out alone. Test audiences found it too bleak and thought Hannah and Selena were certain to be killed soon. Interestingly, this ending was added to the theatrical cut in 2003 as a Post Credits Scene. In the DVD commentary, Danny Boyle and Alex Garland continously stress how the hospital ending is the "real one" and how what became the actual end was the result of very strong reaction from focus groups, leading to the uplifting ending.
* GenreKiller: Together with the American remake of ''Film/TheRing'' that same year, this film effectively read the obituary for the TeenHorror genre of the mid-late '90s, and arguably for the {{slasher|Movie}} genre as a whole. Both films were {{sleeper hit}}s featuring mostly adult protagonists and a lack of the usual slasher tropes that won acclaim from critics and horror fans, and teen horror, which was already on life support by that time, mostly faded out in the '00s.
* KeepCirculatingTheTapes: An increasingly literal example. It’s remarkably easy to find a DVD copy in a used book store these days. Want to find it streaming? Too bad - it was removed from all streaming services in the U.S. in late 2022, and as of [[https://www.joblo.com/28-days-later-4k/ February 2024]], the Blu-Ray version has been rendered out-of-print
* LoopingLines:
** Creator/CillianMurphy rerecorded all of his lines during Post Production, replacing a faux-British accent with his natural Irish accent because he thought the British accent sounded too fake.
** There was quite a bit of ADR going on, according to Creator/DannyBoyle. Several shots were actually set up to accommodate ease of ADR (faces in shadows, the actor out of frame etc) and a handful of new lines were added to otherwise wordless scenes.
* ReferencedBy: [[Referencedby/TwentyEightDaysLater Has its own page]].
* SparedByTheCut: The final cut uses a FocusGroupEnding where [[spoiler: Jim recovers from his fight with the soldiers]], but the director has said that the "true ending" is an alternate cut where [[spoiler: he succumbs to his injuries]]. The latter is included as a DVD extra and is a point of contention among fans and even professional critics.
* StarMakingRole: For Creator/CillianMurphy and Creator/NaomieHarris.
* WhatCouldHaveBeen:
** The original script had a completely different second half, where the focus remains squarely on the four survivors as they hole up in the lab that started the whole mess. It was replaced by the story with the soldiers because of a problematic plot element of replacing every single drop of blood in the human body, in which Jim would offer to save [[spoiler: Frank]] and the scientist would cure him by swapping their blood around. In the storyboards of this ending included as a DVD bonus feature, it's explicitly noted that, given what the film had already established about how contagious TheVirus was, [[FridgeLogic the cure should never have worked]].
--->"And that was the problem right there with this ending. We'd established that one drop in the eye will infect someone. Then how in the flying fuck are we gonna sell the idea that this blood transfusion idea is gonna work? What do you do, clean out every capillary and vein with bleach before making the transfusion?"
** The ending was constantly changing during production. The full ending would have had [[spoiler: Jim having a DyingDream in the hospital, flashing back to being on a bike before the accident. He would have died on the operating table as he was hit by the car]]. Shots of this remain in the theatrical ending. Another ending would have been similar to the theatrical one [[spoiler: with Jim still dying but Selena and Hannah on the farm afterwards]]. Creator/DannyBoyle also considered going for a more ambiguous ending, cutting off [[LeftHanging as Hannah drives at the gates]].
** Creator/EwanMcGregor was the original choice for Jim, but he and Creator/DannyBoyle had a falling-out at the time over the casting of ''Film/TheBeach''. The second choice was Creator/RyanGosling, who had a scheduling conflict.
** Creator/RobertCarlyle was offered the role of Major Henry West. He would later appear in the sequel.
* WritingByTheSeatOfYourPants:
** The original plan was to reveal that the virus had spread worldwide. Midway through shooting, they opted to change things to keep the virus more ambiguous. Since the film was shot almost entirely in-sequence, this is the reason for Sgt Farrell's dialogue theorizing the quarantine. The sequel confirms his theory.
** The entire second half of the original script was scrapped; originally the heroes find that the beacon is automated and there are no actual survivors around, and coincidentally end up at the same research station the infected animals were released from. One scientist is still alive there, and informs them that the infection can be cured by replacing every drop of blood in the person's body with someone else's, leaving them to be infected instead. The crew quickly realized how ridiculous this was ("What do you do, clean out every capillary and vein with bleach?") and quickly came up with the story of the soldiers to replace it.
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