All cast members were required to have their hair trimmed every five days.
In season four, episode five, "Day 4: 11:00 a.m. - 12:00 p.m.", Debbie's cell phone rang with a valid California phone number on the caller ID. Many fans noticed this and called the number; some callers heard a voice mail message telling them that they called the 24 prop phone and to leave a message. Others spoke with various people, including production designer Joseph A. Hodges and stars Kim Raver (Audrey Raines) and Carlos Bernard (Tony Almeida). Hodges later stated in an interview that director Jon Cassar came up with the idea for the "Fan Phone" because the two of them often got bored scouting new shooting locations and wanted a chance to interact with the audience. Hodges indicated they received around eighty thousand calls on the line in the first week. There was also a valid California cell phone number in season three, episode three, "Day 3: 3 p.m. - 4 p.m.", Chase's cell number appeared on Jack's cell.
Roughly twenty-five hours of footage are edited into one episode of the show.
The actors and actresses were not told in advance as to what would happen from episode to episode, so none of them would know whether or not they would be written out of the series.
In season three, it's revealed that Jack received an intricate tattoo on the inside of his left forearm as part of his previous deep-cover operation with the Salazar brothers. Instead of spending several hours in the make-up chair on days when the tattoo needed to be shown, Kiefer Sutherland opted instead simply to get the actual tattoo.