"I'VE always felt that its success is really down to the fact that, like The Sopranos, nobody knows what the ending will be," says Mad Men creator Matthew Weiner.
"That means you have this tension in your gut as you watch."
Dedicated followers of Mad Men, whose sixth and penultimate season begins in less than two weeks, will recognise that tautening of the intestines.
It's what Weiner's show does brilliantly, the writer-producer explaining the omerta that rules on the Mad Men set in Los Angeles thus: "If I'm told how something ends, I literally don't want to see it any more
"So the fact that people have no idea what's going to happen gives us a uniqueness in the marketplace, if nothing else."
You would have thought this was obvious: all the most talked-about television dramas have had hung-on surprises and unanswered questions.
Who killed Laura Palmer in Twin Peaks? Who shot JR? How would The Sopranos (another Matthew Weiner production) end?
But keeping these secrets has become increasingly difficult - especially in the internet age, with its dedicated TV spoiler sites, endlessly recycled gossip and Tweets from cast members, production crews and industry bloggers. Indeed, many spoilers are little more than disguised advance publicity.
"Most shows have press leaks," says Vincent Kartheiser, who plays Pete Campbell in Mad Men.
"Our show works in the opposite direction… we're not going to tell you anything ever… the idea that anything could happen is what brings you into the show. It's quite ingenious."
To judge by the magnitude of the online guessing game about the new season, Kartheiser is bang on the money. "Matthew picks people who have integrity and discretion," says the actor.
"He doesn't hire people who run a publicity-driven career. Most of the actors and writers we hire enjoy the work… it's not about being famous.
"And he gives a little speech before every episode: 'This is our show, this is what we do here, the scripts are confidential, you'll be given what you need for your character, don't talk about it on talk shows, please don't post it on imdb… and if you do these things, there will be consequences'. You do sign papers…"
Storylines are only revealed to the actors the night before the first read-through of each episode - a method that can cause nasty surprises.
I interviewed Jared Harris, the British actor who played Lane Pryce in Mad Men, just weeks before he filmed his character hanging himself from his office door - one of the most shocking plotlines ever on the show.
Unless he was dissembling very skilfully (he is an actor, after all), it was clear during our conversation, in which Harris chatted merrily about being optioned for seven seasons, that he had no idea of his fate.
Weiner says he "shreds everything" and locks the set, yet Mad Men isn't the only show that holds its cards close to its chest. Soren Sveistrup, the Danish creator and chief writer of The Killing, would similarly only present scripts to the actors at the first read-through of each episode.
The identity of the killer was not revealed to the cast until the very last episode - not even to the show's star, Sofie Grabol, who played Sarah Lund.
"On the first season I was obsessed by solving the mystery - and I was very offended as an actor that they wouldn't reveal it to me," said Grabol, who even came under interrogation at home to reveal the culprit's identity.
"My son puts a lot of pressure on me," she said. "Then he starts playing the guilt card… 'I'm your son, you won't tell your own son…'"
And one of the many extraordinary things about the first series of The Killing in the UK - given our access to the internet and that it had already been shown in Europe - was how well kept the identity of the murderer was. No one spilled.
"Well done Denmark… that was a real vote of confidence in that show," says Chris Chibnall, creator of the ongoing ITV whodunit Broadchurch. "I think in general people don't want things spoiled."
Chibnall employed a similar leakage-lockdown on Broadchurch.
"I kept everything secret from the cast," he says. "It gave the production an energy, and the actors a truthfulness, but I also think there has to be a trust between writers and actors and directors to use that method.
"We do have confidentiality clauses, but what you hope is that you create an atmosphere where people want to be part of that secret."
Those agreeing with Weiner, Sveistrup and Chibnall - that a lack of foreknowledge adds to the excitement of a television drama - had better stop reading this article now. Because human nature being what it is - and the internet being what it is - there has, of course, been some leakage concerning the new season of Mad Men.
Indeed many of the (admittedly vague) insights have come from the makers, Lionsgate, themselves.
So what do we know? Well, the first question any fan wants to know before a new Mad Men season is which year it's going to take place in.
This one will be set in "turbulent times", according to Jon Hamm at last week's red-carpet premiere in Los Angeles - and the smart money is on the late spring and summer of 1968, with the assassinations of Martin Luther King and Robert Kennedy, the Vietnam War at its bloody height, students rioting in Paris and Yippies burning up the Democratic Convention in Chicago.
On a lighter note, tight trousers could also give a clue: Hamm has apparently asked for stronger underwear as his late-Sixties strides were leaving little to the imagination).
The music should be red hot, with anyone from Hendrix and Cream to Janis Joplin and the Doors to choose from, although the show has never been too obvious in its song choices.
Don Draper listening to the Beatles' "Tomorrow Never Knows" in season five was worth every dime of its reported $250,000 licensing fee; Megan singing "Zou Bisou Bisou" struck exactly the right note of Yé-yé frivolity; while ending last season with Nancy Sinatra's Bond theme "You Only Live Twice" was masterfully apt.
Weiner himself hasn't been entirely tomb-like, sharing some broad observations with journalists.
And for Mad Men's creator, the unsettling events in 1968 America are relevant to the current zeitgeist.
"For some reason or another, this season feels particularly related to where I feel we are right now, as a country and as a society," he says.
"There's been a bit of a blow to our self-esteem. [There's been] a turning inward as we deal with the loss of something."
And Hamm says that Don Draper is the character most unsettled by the new realities - further pressure, no doubt, on his fledgling marriage to the much younger Megan (Jessica Paré), a modern woman who seems to have no problems moving with the times.
The final episode of season five saw Miss Calvet (as Megan pointedly styled herself, instead of Mrs Draper) beginning her acting career while Don got up to his old ways - being hit on by women in bars.
The last, suggestive words in season five were of a female stranger asking him, "Are you alone?".
Metaphysically, maybe, but physically not, to judge from the few teaser pictures released from the new season of Don and Megan on the beach in Hawaii, Megan looking enigmatically at her wedding ring.
Hawaii? Yes, it seems that Mad Men is going on one of its periodic vacations - possibly for a wedding. According to a journalist from E News! who's actually been granted a preview of the opening episode but is keeping politically (and contractually) schtum, it will indeed feature a wedding.
Oh, and two deaths. Whose? Who knows, or wants to know? Fans of the show's wittiest character, the dry-as-a-martini Roger Sterling (John Slattery), will be hoping the show's resident silver fox isn't for the chop.
Pete Campbell will survive - or at least he won't commit suicide, as some fan forums were predicting after his torrid last season, in which Mad Men's least likeable character entered a full-blown midlife crisis.
"I know the character of Pete very well," says Weiner, "and I don't see Pete Campbell as someone who would ever commit suicide."
Weiner has also been quick to reassure fans of Peggy Olson (Elisabeth Moss), who were upset when Peggy left Sterling Cooper Draper Pryce, assuming she would no longer be part of the show.
"I never said to anyone that Elisabeth was leaving the show," says Weiner.
"I just said that the character was moving on and you'll have to watch. That's what I always say: 'You'll have to watch'."
And then which character will be the first to drop the f-bomb? That's right - although swear words have been used in Mad Men before (Megan got to use some particularly fruity French-Canadian slang in the last series), the show is relatively obscenity-free compared to such HBO shows as The Sopranos and Deadwood. HBO famously turned down the show, leaving it to swear-free, nudity-forbidding cable rivals AMC, who have now, it seems, relaxed their stance.
Finally, what clues can be gleaned from the intriguing poster for the new season - a sketch showing Don Draper, or rather Dick Whitman to give him his real name - passing his doppelganger in the street?
Is Dick/Don's past catching up with him? Or perhaps something deeper? After Roger Sterling's LSD trip in season five, Don wouldn't be the first Mad Men character to have an out-of-body experience.
Either way, this is definitely the penultimate series; Weiner has confirmed that it's seven and out for his creation. One of the most tantalising utterances he has made is his admission that he knows precisely how Mad Men will end.
"Let's just say that I have a picture in my head of what happens in the final scene of season seven," he has said. What can that mental image be? Don Draper in flares? Joan burning her bra? Glen Bishop burning his draft card? But as Vincent Kartheiser says when I ask him whether he'll be growing his hair in the new season,
"You'll have to wait and see, my friend… you'll have to wait and see."
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