The 24 cast has varied every season. The changes have ranged from small to almost the full cast. The only actors who appear in each of the six seasons are the main star, Kiefer Sutherland, and Glenn Morshower (Aaron Pierce, Special Agent with the United States Secret Service; a relatively minor character until Season Five).

The show has always had many guest stars not billed as main cast members, as well as Special Guest Stars from time to time. Special Guest Stars are usually former cast members making a return appearance, which can last from a single episode to almost an entire season, as do some guest stars. However, some special guest stars have been new characters usually played by well known film and television actors.