So if Kiefer Sutherland signs a three year deal or whatever, you know he's can't die for at least 3 more seasons?? I can't see the suspense in watching something like that...
Not necessarily. Generally all of these contracts are very one-sided and the actor needs to commit to the show for the contract period, but they can be killed off anytime during that period. All of the stars from the pilot episode of 24, were contracted for 3 years(inc. Teri who was killed off). All of the stars from Lost, who were in the pilot, who have been killed were contracted until the end of Season 3. All of the stars from the pilot episode of Alias were contracted for 5 years. Most shows are the same (although it's a lot more common when shows are first starting and the relatively unknown actors committing to the pilot).
But since Keifer is one of the big guns behind 24, then he would be able to negotiate his guaranteed survival quite easily.
The shows can do it quite easily - "Not interested in working? Fine, we'll find someone else who is". You'll find most actors from TV shows (Keifer, would be one of the few exceptions), will never turn down a job.