Sarah Clarke wasn't aware her character was a mole until well into Season 1.
Gregory Itzin wasn't told his character was the Big Bad of Season 5 until the episode it was revealed. Itzin has said in interviews he'd played President Logan as a well-intended, but incompetent good guy up til that point.
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.
Deleted Role: Photos from the filming of season 8 showed Glenn Morshower on the set with Cherry Jones, suggesting that Aaron Pierce originally was supposed to make an appearance but his scenes were deleted.
Development Gag: The wedding side story in season 2 was a nod to the original concept that 24 be a comedy about preparing for a wedding.
Development Hell: A feature film adaptation of the series has been in the works for years.
Dyeing for Your Art: For continuity reasons, the cast of at least the first season had to have their hair trimmed every five days.
Executive Meddling: The reason Keeler survives the crash and subsequently disappears was thanks to a network mandate that the writers couldn't kill off a sitting President, as was originally intended in the Air Force One attack.
Fan Nickname: In season 8, Jack himself was given the nickname, "DarthBauer◊". Interestingly, when one would call him this nickname, he can be considered as the Darth Vader to Kim's Princess Leia.
Curtis Manning was called "Black Bauer" or "Black-Jack" for effectively being just as badass as Jack, and for being... well, black.
Fans of Larry Moss in Season 7 affectionately referred to him as "Boss Moss".
The bag of equipment Jack can sometimes be seen toting was known as his "Jack-pack."
Television Without Pity had a cottage industry of these, at least in the first few seasons' worth of recaps. Some of the more notable ones include "Bitchelle" (Michelle Dessler), "Soul Patch" (Tony Almeida), "Spawn of Kiefer" (Kim Bauer), and "ImhoTerror" (Habib Marwan).
"Sparky" for Paul Raines, after Jack used electricity to torture him.
Fast Karate for the Gentlemen makes a habit of creating these for pretty much every character not in the core cast, and even one or two who are. Special mention goes to "Special Agent Babykins" (Owen), "JibJub" (Prescott O'Brian), and the "President of Not-Iran" (Omar Hassan).
I did 24; it was terrible. I hated every moment of it. Kiefer was the most unprofessional dude in the world. That's not me talking trash, I'd say it to his face. I think everyone that's worked with him has said that. I just wanted to quit the business after that. So, I just sort of stopped.
Irony as She Is Cast: Live Another Day has Stephen Fry playing UK prime minister Alastair Davies, who according to Word of God is a member of the Conservative Party. In real-life Fry is an outspoken liberal, who has been openly critical of the depiction of torture on the show.
Jossed: Once Teri Bauer began showing shades of being an Action Girl, a theory popped up that she was a former CTU agent, or worked with another intelligence agency. Aside from the fact that this almost certainly would have come up during the season, it's thoroughly Jossed by the tie-in comics; she was and always had been a civilian with no intelligence background. She was just a badass.
Keep Circulating the Tapes: Although the entire series is on DVD and can be streamed on Amazon, the "Previously on 24" recaps that originally appeared in the original airings of the first season have not been used again and have not been featured in any reruns, home video releases, or streaming services. It's extremely odd since the recaps from all other seasons are featured in reruns and home releases.
Disclosed memos from within the former Bush administration suggest that, if 24's torture methods didn't inspire the initial events at Abu Ghraib Prison and Guatanamo Bay Prison Camp, they at least inspired their justification and continuation to this day.
On a less controversial note, the suspects of the Abu Ghraib abuses claimed 24 as inspiration for their tactics, and military intelligence instructors have complained to reporters that they have to drill 24 and its depiction of torture out of the heads of new students.
On a much less controversial note, the positive portrayal of David Palmer is at the very least credited (which the writers deny despite actor's Dennis Heysbert's approval) for the election of the the first African-American President.
Mid-Development Genre Shift: This was originally a much different show than what it ended up becoming. Prior to the first season, the producers had envisioned the show as a romantic comedy with dramatic overtones about the planning of a wedding. This was changed in pre-production to a government agent (originally named Jack Barrett) trying to stop the assassination of a senator. The "wedding" angle would be revisited in the first few hours of the second season with the Warner family.
Ramon Salazar; the psychopathic terrorist dabbling in bioweapons, is not nearly as crazy as...Ramon Salazar; the psychopathic terrorist dabbling in bioweapons in Resident Evil 4.
Old Shame: James Cromwell does not look back on his tenure on the show with much fondness. He hadn't seen the show beforehand and only took the job at the urging of his agent, claiming it would make him more-well known, and because he was being offered a lot of money for it. He hated how the show glamorized torture for information, and hated playing a character with no redeeming qualities.
This view is actually shared by the writers too, as Cromwell happened to guest star in Season 6, widely considered the weakest of the show.
The Other Marty: Margo, the antagonist of "Live Another Day," which was originally supposed to be played by actress Judy Davis. As principal photography began though, Davis was forced to drop out because of family concerns, so the role was recast to Michelle Fairley.
The seventh season ended with a large number of plot threads being resolved (including Tony Almeida confronting, and Renee Walker presumably killing, the mastermind behind the Myth Arc of the last four seasons) and bringing things full circle with Kim Bauer returning to save Jack from a weaponized virus via a transplant. There was lots of uncertainty for a time, but the show was unexpectedly renewed and brought back for another year. In response, the producers moved the show all the way to the other side of the country (New York), introduced an entirely new cast of characters, reworked a previously-good supporting character into the season's Big Bad and indicated that the previous mastermind, Alan Wilson, paid for his crimes at the cost of Renee's career.
Ratings for season 8 suffered and the show was not renewed as a result. However, the last stretch of episodes were highly praised for shaking up the series' status quo dramatically (and ended with Jack on the run and President Taylor disgraced and forced to resign from office over her role in the Russian treaty). A year-and-a-half later, it was announced that the show would return in a limited series called 24: Live Another Day. Three years later, another limited series called Twenty Four Legacy (which doesn't feature Jack Bauer or any other returning characters, except Tony) began in February.
Promoted Fanboy: Leighton Meester appeared in a couple of Season 4 episodes. She was a huge fan of the show and was ecstatic to hear that her character would get killed off.
Real-Life Relative: The actors who played George Mason and Nina Myers married between the first and second season, making the interrogation between them in the latter rather interesting to watch.
Role Reprise: As it takes place between Days 2 and 3, but was released during the run of Day 5, a lot of people whose characters had been killed or written out of the show returned for The Game. Such as Sarah Wynter (Kate Warner), Paul Schulze (Ryan Chappelle), and Zachary Quinto (Adam Kaufman).
Shrug of God: The writers have not settled on what the ultimate fate of Wayne Palmer was after the events of Day 6. A newspaper prop in 24: Redemption viewable by freezing the movie indicates he died sometime between the sixth season and the movie, but the writers have stated that for all they know, he might still be alive. As of April 2014, he's labeled as "Deceased" on the 24 wiki.
Throw It In!: Aaron Pierce addressing President Logan as "Charles" at the end of his "The Reason You Suck" Speech was an ad lib by Glenn Morshower. Gregory Itzin's stunned reaction was the real thing and it was kept in because of how awesome and fitting it was.
Trope Namers: 24 has named the following tropes on this wiki:
Jack Bauer Interrogation Technique: The use of force or downright torture during interrogation. No less than 67 scenes involving interrogation were counted in the first five seasons, many of them involving the inflicting of physical pain, and mostly involving Jack Bauer.
"You probably don't think that I can force this towel down your throat. But trust me, I can. All the way. Except I'd hold onto this one little bit at the end. When your stomach starts to digest it, I pull it out. Taking your stomach lining with it. For most people it would take about a week to die. It's very painful."
Trapped by Mountain Lions: invokedA subplot with no relation to the main storyline that is often used as Padding to give one or more characters something to do on-screen. Named for a subplot in season 2 where Kim Bauer is trapped in the wilderness by a mountain lion, an event that is often held to be irrelevant to the rest of the story.
Uncredited Role: Harris Yulin was not credited for his role as Roger Stanton in the second season. Due to some disagreements with the producers over his credit (Yulin wanted his name to be the sole credit when it came up during the Guest Stars credits and not have it shared with another actor's credit, but was refused), leading to him requesting to have his name removed from the credits of each episode he appeared in.
Writing by the Seat of Your Pants: The series was notoriously written on the fly, with the writers starting each season with practically no concrete idea where the thing was going to end up. Notably averted by Season 7, due to the 2007 Writers Strike and a year-long delay, resulting in a much more cohesive, planned-out storyline for that season.
The actor who played a mosque door greeter in season 2 also played minor season 4 villain Tomas Sherek.
Tony Todd played the detective investigating the death of Alan Milliken in season 3. He then came back season 7 to play General Juma.
Michael Bryan French, who played one of the Secret Service agents interrogating Jack during the first season, went on to play a completely different Secret Service agent helping out against Juma's White House invasion eight years later in the seventh season.
Averted with Adoni Maropis. He filmed scenes for one episode of Day 4 as the father of the two Arab-Americans Jack and Paul teamed up with before later playing Abu Fayed, one of the main antagonists of Day 6. However, his scenes for Day 4 were deleted (though can be viewed on the DVD set), leaving Fayed as his only character to actually appear in the series.