Kiefer Sutherland, 24
Tonight at 9 pm/ET, Fox's 24 tackles the sixth hour in Jack Bauer's latest daylong frenzy. Will the world's most resilient terrorist-buster get closer to the heart of this season's criminal conspiracy? Will he once again save the day? And in doing so, will he ever break for a sandwich or "see a man about a horse"? Your guess, you ticking-clock-loving viewer, is as good as anyone's. As series star Kiefer Sutherland and executive producers Howard Gordon and Evan Katz reveal, one of the show's biggest secrets is that, at the end of the day, no one is quite sure what will happen at the end of the day.

"We're lucky if, at the beginning of the [season], we know where it starts," says Gordon, half-jokingly. "It really has become much more of an improvisation, but I think that's part of what gives the show its energy. We paint ourselves into corners and blast our way out." Adds Katz, "For instance, last year we didn't know that Jack wouldn't wind up dead. We knew in general that we'll avert a terrorist threat or conclude the conventional story, but we begin the year sort of pretending we have the first four to six [episodes figured out]. But even as we're writing those, they change. Things change too quickly to plan ahead too much."

As a result, Sutherland has to feel for the show's scribes. "As freeing as the real-time format has been for the actors, it has been equally restrictive for the writers," he notes. Fortunately, over the first four seasons, 24 has discovered a certain rhythm, a pattern by which the "day" is more or less broken up into three acts. "Literally, the first eight episodes of any specific season have taught us how to do the next eight and then the next eight," says Sutherland. "As soon as we all trusted and accepted that, our lives got a lot easier and the shows got better. We now have some faith in the way we figure this out."

The inherently unpredictable nature of 24 is what keeps Sutherland coming back, day in and day out, always anxious to take Jack to new places emotionally. "This show has taught me about working with minutia and making 15 small changes over the course of one season to another," he observes. "The unique aspect for me this year is that Jack does not work for the Department of State. He does not work for CTU. Everything he is doing this year is an emotional response to a series of events, it is not about duty or a sense of responsibility. Those are all small changes, and to inject those into this character and thus separate him, on a very small level, from last year or the year before, is an incredible challenge and something I enjoy immensely."

Of course, dealing with such intense subject matter — bombs, virus outbreaks, shot-down Air Force Ones, presidential-assassination attempts, whatever Mandy is up to at a given moment — could easily cast a pall over the 24 set. Fortunately, the mood is oft lifted by one major on-set practical joker. "Carlos [Bernard, Tony] is your guy," says Sutherland. "He had two police officers arrest Mia Kirshner [then playing Mandy] for allegedly smoking marijuana outside her dressing room. And as they went to take her away, Carlos got into a fight with the officers about [how they really should be] protecting the city."

Recalling another mood-breaker, Sutherland says, "There was a moment in Season 4 when I'm climbing up the side of the Chinese consulate and my gun falls out of my pocket and goes off and shoots me in the ass. Anytime we get to have a laugh, yeah, we take it!"

Something that makes the occasional viewer grin — particularly those schooled on Los Angeles traffic — are the storytelling leaps all but demanded by 24's real-time structure. (Drop by any message board and you'll see heady debates over just how long it really takes to get from Bakersfield to Downtown L.A.) "We know we take liberties," says Katz, citing the swiftness with which federal agents swarmed an airport during this season's fourth episode, "but CTU is clearly very efficient and effective. Frankly, we're honored and lucky to be working with them."

In the same jovial tone, Sutherland says he has labored hard and often for Jack to, you know, eat a little something during his breathless adventures, but thus far has been forced to starve. "Every year, I [as Jack] have eaten at least once, and [series creator] Joel Surnow has cut it out," he reveals. "I even used the bathroom once, and Joel cut that out. I have fallen asleep while working surveillance in a car, too, and he's cut that out.

"You really have to take that up with Joel," says the actor. "We've tried!"