- The first season was only fully plotted for 13 hours, because the producers weren't guaranteed a full season. When the series was given a full 24 episodes, new plotlines were drawn up for the rest of the 24 hours (including Victor Drazen - The Man Behind the Man, the reveal that Nina Myers was an enemy, and Kim's second kidnapping).
- The actor who played Kevin Carroll (the false Alan York in the first season), Richard Burgi, was originally cast in the role of Jack Bauer. He would have had that role if they hadn't been able to get Kiefer Sutherland. Once Sutherland was brought on board, Burgi accepted the role of Carroll.
- Alberta Greene was originally intended to be played by Alberta Watson in the first season (the part was named for her). She wouldn't join the series proper until Season 4, playing a character who ironically turns out to be just as "by-the-book" as Greene.
- Nina Myers was originally intended to be a loyal agent, but her motivations were changed after several episodes had already been filmed with her "good" persona. The scene where Jack is ordered to shoot her at the side of the road would have been a permanent death, but this was changed to keep her alive (via the reveal that Jack had given her a flak jacket).
- The most well-known example: Teri Bauer was originally supposed to survive being shot at the end of Season 1. The film crew shot three different endings: the broadcast version (where she dies), one where she is shot but pulls through, and one where she is unharmed and walks off smiling with Jack. Teri's fate was up in the air until the start of the second season, but this is a case of Executive Meddling that turned out for the best: the incident helped develop Jack Bauer as a character and firmly established 24 as a tragic series where truly Anyone Can Die.
- During Season 1, Teri Bauer was going to fall asleep for a few hours. Producers insisted she remain in the show, resulting in an extended subplot where she contracts amnesia.
- At one point, Joel Surnow wanted to acquire the rights to The Da Vinci Code in hopes of working it into a plotline for the show's third season, but Dan Brown rejected him. How he would have worked Jack Bauer into a story about Leonardo da Vinci, Pagan cults and Jesus' family is anyone's guess.
- Gabriel Macht was originally supposed to play Chase Edmunds in the third season. However, the producers were so impressed with James Badge Dale's audition that they hired him instead, and told Macht he would get a significant role in the next season. This never materialized. Chase was also originally slated to die as a result of a botched hand-reattachment operation at the end of the season, but this was scrapped in the script stage.
- The producers toyed with the idea of casting Donald Sutherland as Jack Bauer's father, Phillip.
- Originally, Ryan Chapelle was originally going to fake his death (and come back later) as a result of Stephen Saunders' demand that he be killed. However, after the script leaked online to a message board, the script was changed to kill him off permanently.
- Mandy was supposed to show up in Season 5, but ended up being removed from the narrative. It's believed she would have been the assassin who kills Palmer in his hotel room (replaced by a male assailant). Additionally, the producers went through two different male actors for the role of Conrad Haas (the assassin) in the premiere before settling on Jeff Kober, as at one of the previous actors who played the character was "too young". The 24 TCG shows a different actor on the trading card of Conrad Haas, likely one of the original actors to play the role.
- A well-known example: Glenn Morshower (Agent Aaron Pierce) was originally slated to die in the fifth season, but he blew the producers away with his ad-libbed response of "Is there anything else, Charles?" during a pivotal scene with President Logan late in the season. The writers hastily changed the scripts to keep him alive.
- The Season 6 DVD commentaries reveal that Josh was intended to be Jack's son, but it was changed at the last minute to have Josh be Philip Bauer's son instead. Philip was originally going to reveal this to Josh when they were on the oil rig, but it was scrapped because it would have added too much to an already-dramatic situation.
- According to various sources, Tony was at one point intended to make an appearance in the sixth season finale. It got as far along as the planning stages, with various scenarios being rumored (one would have had Tony get a separate scene after Jack walks out of Heller's house, and another would have had him confront Jack either in the finale or a DVD-exclusive scene). Either way, it never got far enough along to be scripted and filmed, although it likely would have been a better surprise than the reveal in the Season 7 trailer (for a season that was delayed for a year because of the WGA Writer's Strike, leaving fans up in the air).
- It's almost a Running Gag the number of times Carlos Bernard managed to talk his way out of being killed off. There were at least four serious bids to kill Tony (when he was shot in the neck in Season 3, when Mandy detonated the car bomb in Season 4 [which, once it was decided that he would survive, was planned lead instead to Michelle's death, suicide from grief], when the second car bomb went off in Season 5, and when Christopher Henderson stabbed him with the syringe in Season 5). Ironically, the only one of these he did not object to, feeling it was a fitting end to the character that supplemented Jack's character arc, was later Retconned.
- Season 6's Darren McCarthy was initially cast as Eddie Izzard, but he was forced to cancel due to scheduling reasons.
- When a movie was still in the works, Tony Scott was considered to direct it. His suicide was one of the factors that ended the project.
- Jennifer Aniston, a big fan of the show, wanted to guest star. Producers turned her down, saying it would spoil the illusion. As she was so famous, they'd see her as Jennifer, rather than the character she played.
- An early photo shoot for Day 1 featured Mia Kirshner alongside the other five leads, meaning Mandy initially was meant to have a much larger role, possibly the sixth lead, than she did.
What Could Have Been / 24