A whole lot has happened on "24" this season.

A train has been sabotaged into a deadly wreck. The Secretary of Defense has been kidnapped by terrorists and put on Internet-televised trial.

Those terrorists have caused a nuclear power plant to melt down and have threatened every other nuclear facility in America. They've shot down Air Force One, critically injuring the president and killing his son.

They've also stolen a nuclear warhead and the launch codes necessary to detonate it.

How much more can happen?

And is there anything "24" can't survive . . . like maybe killing off its main character, Jack Bauer?

"In theory, if there comes a point in the show where to kill Jack is what services the show best, then I think we've all understood that that's something that everybody's prepared to do — myself included," said Kiefer Sutherland, who not only plays Jack but is credited as a co-executive producer on the show. "And I think every actor who has come on the show has felt that way."

Certainly Sutherland is the only actor who wasn't at least somewhat expendable as the show began its fourth season in January. Jack was surrounded by a whole bunch of characters we'd never seen before, with the regulars from past seasons having been either killed off (Sherry Palmer) or written out (Jack's daughter and President Palmer).

Some have reappeared — Tony Almeda and, this week, former President Palmer — for a few or a lot of episodes, but only Jack has been in every episode of all four seasons of the show.

On the one hand, Sutherland made it clear he wishes his former co-stars were still on the show. "So, yes, for very selfish reasons, I miss them a great deal. But I think it was absolutely the right thing to do for the show."

And he insists he wouldn't hesitate to exit "24" himself if it was the right thing to do.

Not that he's tired of his role. He said the "biggest surprise" he's had is that he'd "still be enjoying it" in the fourth season.

"One of the things that I had heard from other actors who had done television before . . . was that they got frustrated playing the same character over a long period of time," Sutherland said. "I've actually found it to be the opposite. I think it's probably one of the most challenging things I've ever had to do.

"The writing is such that it is always exciting, but the great challenge is to try and stay fresh and find, hopefully, another layer of subtext within the text for that character to play on. . . . I'm not even close to mining all of the potential things for Jack to experience."

And the show's executive producers certainly don't seem in any hurry to replace Jack. Joel Surnow said he was just "going to pretend I didn't hear the question."

His partner, Robert Cochran, allowed that "anything's possible. But if there's anything that's impossible, that's it."

THE PRODUCERS OF "24" INSIST they're still flying by the seat of their pants as each season goes on — that they don't plot out 24 episodes at a time and don't necessarily know where any season will end up when it begins. Or even when they're three-quarters of the way done writing it.

"We still do it," said executive producer Robert Cochran. "It's honestly not something we choose to do. It just seems to be the way it happens."

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E-mail: pierce@desnews.com