The series follows the fortunes of a group of plane crash survivors who are stranded on a mysterious island somewhere in the South Pacific, a place where a polar bear might be found beneath the palm trees and invisible man-eating monsters give visitors a special welcome. What’s more, our castaways aren’t exactly average Joes, encompassing the whole gamut of liars and thieves, sages and princesses. Quite what Desperate Housewives’ neurotic Bree Van De Kamp would make of this rabble is unclear, but she certainly wouldn’t have liked all the noise.

On set in Hawaii, it’s fair to say that Lost’s cast is flagging. The show has taken America by storm, but a seemingly endless succession of jungle chases, boar hunts and fisticuffs has taken its toll. The British actor Naveen Andrews (The Buddha of Suburbia, The English Patient), who plays the Iraqi Sayid Jarrah, has been pelted with so much freezing-cold artificial rain that he is borderline hypothermic. Matthew Fox (Party of Five), who plays Jack, the resident doctor and hero, is also battle scarred. “It’s as demanding as hell. It can be really rough when I get beaten up.” Fox’s on-screen squeeze, Evangeline Lilly, aka the doughty Kate, has been pounding treadmills for two hours after shooting to get her body temperature back to normal. The setting may be paradise, but the filming is not.

The Lost campus consists of 500 acres, including several jungle sets and a meandering strip of beach. It’s a prime surf spot — head-high rollers buffet the shoreline, and everything would look postcard-perfect were it not for several tonnes of charred aircraft fuselage on the beach. The characters’ campsite, nestled in a gentle cove, is strewn with aircrash detritus and makeshift tents. At the moment, all of the 14 main cast members are gathering for a crucial ensemble shot. I am warned on pain of death not to tell anyone what I see. This becomes a recurring theme over the next few days — in Lost almost every scene contains some kind of plot twist or cliffhanger. It’s this succession of whos, whats and whys that makes it so addictive.

It’s not just the fans who get hooked, either — the cast seem to be just as engrossed with their own show. “They don’t tell us anything,” says Andrews. “In the beginning I thought how exciting it would be if the character is discovering things at the same time as the audience. But now I’m frustrated at wondering what happens. Is there a plan?”

Andrews’s fevered inquisitions mimic the show’s fansites, where conspiracy theorising about what was going on reaches epidemic proportions. I put it to Dominic Monaghan (The Lord of the Rings), who plays the has-been rocker Charlie, that maybe the writers don’t know what’s going to happen either. “I started off thinking the same thing. But I think that they do. Damon Lindelof [one of the show’s creators] said to me that he had a five-year plan for the show. He talked in the long term about a few things. In terms of why we’re all tied together and what this island represents, I have a hard time wondering if they know or not. But they have surprised me again and again when I’ve thought: ‘This is going to be terrible,’ and it ends up being great.”

That is a neat abridgement of Lost’s weekly party trick — offer up a plot twist so fantastical that it lurches towards being absurd, and then, through a mix of taut scripting and a carefully cultivated sense of the surreal, make it believable. This kind of U-turn takes place, on average, at least three times an episode.

The supply line for all these surprises is twofold. First there are the weekly flashbacks. In each show, one character has their back-story filled out in retrospect — we learn who they are and what they were doing on the flight in the first place. It is a technique that creates a cache of secrets, motives, answers and, yes, more questions to throw into what is already a boiling pot of intrigue.

And second, there is the island itself, complete with all sorts of he’s-behind-you ghouls that may or may not exist. Monaghan has his own ideas as to what the beast looks like. “When I’m running away from it, I think of an African elephant with 17 cats stuck to its back.” If that’s an example of what Lost is doing to the heads of the cast, imagine what it is going to do to you.

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