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In season 3, 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 hours in the makeup chair on days when the tattoo needed to be shown, Kiefer Sutherland opted instead simply to get the actual tattoo.

The actors are not told in advance as to what will happen from episode to episode so none of them know whether or not they will be written out of the series

All cast members were required to have their hair trimmed every five days.