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Kiefer Sutherland has said in many interviews that he is not the star of the show: the time element is.

Partway through season 2, Kiefer Sutherland injured his leg while exiting his trailer, so a small scene was written in where Jack Bauer must pull a piece of shrapnel from his leg after a plane crash to explain Sutherland's limp.

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