- Actor-Inspired Element:
- Jim was written to be British but Cillian Murphy struggled to do an accent - so they let him use his natural voice. London has a high number of Irish immigrants anyway.
- Naomie Harris worked with Danny Boyle 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 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.
- Career Resurrection: After the triple failures of A Life Less Ordinary, The Beach and a straight-to-TV Awful British Sex Comedy called Vacuuming Completely Nude in Paradise, many people assumed that Danny Boyle would be another flash-in-the-pan 90s Indie director. Oh, how they were wrong.
- Cast the Expert: 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.
- Dyeing for Your Art: Christopher Eccleston and the rest of the actors playing the soldiers did a three day boot camp to prepare.
- Fake Brit:
- Frank is played by Irish actor Brendan Gleeson.
- Scouser Megan Burns attempts a London accent for Hannah.
- Genre-Killer: Together with the American remake of The Ring that same year, this film effectively read the obituary for the teen horror genre of the mid-late '90s, and arguably for the slasher genre as a whole. Both films were sleeper hits 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.
- Looping Lines: Cillian Murphy 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 Danny Boyle. 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.
- Spared by the Cut: The final cut uses a Focus Group Ending where Jim recovers from his fight with the soldiers, but the director has said that the "true ending" is an alternate cut where 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.
- Star-Making Role: For Cillian Murphy and Naomie Harris.
- What Could Have Been:
- 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. Essentially Jim would offer to save Frank and the scientist would cure him by swapping their blood around.
- The ending was constantly changing during production. The full ending would have had Jim having a Dying Dream 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 with Jim still dying but Selena and Hannah on the farm afterwards. Danny Boyle also considered going for a more ambiguous ending, cutting off as Hannah drives at the gates.
- Ewan McGregor was the first choice to play Jim. Ryan Gosling was the second.
- Robert Carlyle was offered the role of Major Henry West. He would later appear in the sequel.
- Writing by the Seat of Your Pants:
- 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.
Trivia / 28 Days Later