The cast of Mad Men on the hit show’s sixth series

But then, as Pare says at the LA premiere of the sixth series: “That would be the most boring TV show in the world.’’ Fortunately for fans, the new series promises to be anything but.

Set in the hubbub of late 1950s/early 60s New York, Mad Men centres on enigmatic advertising executive Don Draper (John Hamm), a partner in a cut-throat agency, Sterling, Cooper, Draper, Pryce. From the off, Don has tried to escape his past life as Dick Whitman, the son of a prostitute and a father he cares little about. Since moving to New York, self-made man Don has adopted a new identity (and until the third series kept his real name and background a secret from his first wife, Betty) and now earns his keep writing seductive advertising slogans.

The third series saw the breakdown of his marriage to immature Betty (played by January Jones) with whom he has three children, while the fourth and fifth showed a softer side to Don when wannabe actress Megan vied for his heart.

While Don’s home front might seem rosier – though the fifth series picked up on the tension between idealistic Megan and her troubled husband – there’s no chance of the ad man jumping for joy in series six. “Don is dealing with demons that he has dealt with for a long time in his life,’’ says Hamm. “I think that is part of his journey and [in this season] we realise more and more why this guy got to where he’s got and why he is how he is. The more we learn about his past, the more we realise he’s a pretty damaged guy.’’

Don’s personal life may be laden with woes but Sterling, Cooper, Draper, Pryce is on the up. At least it was until anguished accountant Lane Pryce, played by the late Richard Harris’s son Jared, killed himself. And then there was that small matter of office matriarch Joan Harris sleeping with a wavering client to secure his custom. “When we last saw the firm, things were looking up,’’ says Hamm.

“We had issues with how Joan was treated and the unfortunate passing of Lane but it was an upward trend. We’ll see if that continues.”

Elisabeth Moss, who plays rising copywriter Peggy Olson, is certain the melancholic tone will pervade series six. As Don’s protégée, Peggy started her career as a secretary before climbing the ranks as a writer. Towards the end of the last series, Peggy left the safety of the agency – and her long-time mentor Don – to join a rival company.

“I don’t think this season’s going to get any lighter,’’ says Moss, who worked with Keira Knightley in Lillian Hellman’s play The Children’s Hour. “If anything, this season just goes a bit more in depth into why these characters are so dark.’’

For Alison Brie, who plays Trudy, the wife of head of accounts Pete Campbell, there is one journey she’d like her character to go on – one that would certainly offer light relief. Only it doesn’t involve slimy Pete, who has irked many of his workmates during the series. “Oh man, if it were up to me Trudy would end up in the arms of Don Draper!’’ jokes Brie.

Unlikely as it is that Brie will get her wish, she is optimistic that her character will stay with Pete. The couple, who became parents in series four, struggled with their new life in the suburbs and, at one point, Pete had an affair. “I always think there’s hope for Pete and Trudy because Trudy really loves him,’’ says Brie. “She doesn’t know about all of the bad things that he does.’’

Trudy might be kept in the dark, but off screen writer Matthew Weiner is also giving nothing away. One thing he will say, though, is that there’ll be comedy in series six. Humour has been weaved throughout Mad Men, with one memorable scene in series three showing a tipsy secretary straddling a lawnmower in the office before running over and maiming a visiting businessman.

“The tone in this season is very internal,’’ says Weiner. “It has all the tension we usually have but I also think it’s very funny. I think it’s easy in a serious environment like this to forget that you’ve got to entertain in every way.’’

Weiner, who has a framed letter from President Obama on his wall praising the series, is astounded by the way the programme has taken off.

“I’m floored by the success,’’ he says. “I told them [the TV commissioners] that it would be a success because I’m a salesman but I really never expected it to happen.’’

Hamm is similarly bowled over by the reception the show has had with its global audience: “It’s amazing to me that what seems to be a very specific story about a specific period in American culture has resonated throughout the world. There’s just something global about the way men and women deal with each other and personal lives, private lives and professional lives moulding together and I think that speaks well in many languages.’’

The scripts and the internal battles surrounding Mad Men’s complex characters might account for a huge chunk of the show’s appeal, but for fashion fans the coveted clothes that the characters wear are just as vital. Indeed, even the actors get a little doe-eyed around the glam garments.

Christina Hendricks, who stars as Joan, would love to get her mitts on one male workmate’s threads: “This year my wardrobe envy might be Harry Crane. He’s got some pretty great outfits.’’ (Harry is famed for his thick-rimmed spectacles and nifty dickie bows.) For Hamm, who starred in hit film Bridesmaids, his devotion to Mad Men might wane were he required to dust off his flares and take Don into the 70s. “I don’t know if Mad Men gets to the 1970s,’’ he says. “Hopefully we won’t. I lived through the 70s and I’d rather not get into those clothes again.’’

• In yesterday’s feature on Edinburgh Zoo, two picture credits were omitted in error. Photograph 3 was by AH Baird and photograph 5 was courtesy of Edinburgh Zoo.