The Downton Abbey 'phenomenon'

British period drama Downton Abbey has become a global sensation since hitting our screens in 2010. Now the hit ITV1 show is back with a third series set in the roaring 1920s with a new Oscar-winning star.

Perhaps unsurprisingly, Downton Abbey writer and co-creator Julian Fellowes feels a bit like he is "riding a whirlwind".

An almost instant hit in the UK, series two cemented Downton as a worldwide "phenomenon" and one of the UK's biggest exports, with more than 100 countries owning the rights to broadcast the show.

The American audience has taken the Crawley family's antics to their heart. Later this month, the show will battle TV drama giants such as Mad Men and Homeland as it competes for 16 awards at the Primetime Emmys.

Fellowes is both pleased and "overwhelmed" by the show's success stateside. While he has "no idea" what the secret is, he thinks Downton's "modern pace" compared with other costume dramas is key.

"When you are watching one of those old ones on DVD, you have to sort of calm down. You say to yourself 'Now this is a long ride,' whereas I don't think with us you do," he says.

"It looks like a traditional, lovely, classic British television series, with lovely costumes and houses and all that stuff.

"But actually it has the rhythm of a more modern show, with lots of different stories happening at once and you have to concentrate.

"You can't just wander off and come back because you might have missed half of one whole story. I think that combination sort of seduces you in one way but holds you in another."

The cast is full of praise for Fellowes' writing talents. Jim Carter, who plays butler Mr Carson, one of a handful of Downton stars in the running for acting Emmys on 23 September, says: "It's the age old thing, people like stories.

"The fact that it's period, I mean it could be set in space. If you've got good characters and good stories, people will follow them."

Writer Julian Fellowes joined stars Elizabeth McGovern and Hugh Bonneville at this year's Golden Globes, where Downton added to its haul of awards

Downton's mass appeal has already translated into awards from Bafta, the Emmys and the Golden Globes.

Now US casting directors are starting to take notice of cast members such as Dan Stevens, who plays "cousin Matthew" Crawley.

He makes his Broadway debut next month in The Heiress, alongside The Help's Jessica Chastain. "It's amazing," says the actor.

"It's the sort of artistic aspiration of a lot of people to crack America and it was never the intention of any of us to do that when we started out."

His character's will-they, won't-they love story with Lady Mary, played by Michelle Dockery, was central to the first two series.

But after the 2011 Christmas special ended with them canoodling in the snow - which "took long enough", according to Dockery - series three begins with love's young dream planning their nuptials.

Stevens drops some heavy hints there are still "thrills and spills" to come. "As anybody who's married knows, it's not all plain sailing after the wedding either," he says.

Hollywood hits Downton

Much of the excitement surrounding series three centres on the arrival of Oscar-winner Shirley MacLaine, playing the mother of Cora, the American Countess of Grantham.

As a rival to Dame Maggie Smith's matriarch, the Dowager Countess, we can expect sparks to fly, according to Stevens.

"It's a terrific dynamic. The classic Dowager Countess one-liners are almost doubled back by having a presence like her on set and they sparked off each other incredibly well," he says.

By all accounts, MacLaine is a "huge fan" and was "a sensation" on set, entertaining the cast with non-stop stories from her movie and Broadway career.

The third series sees the residents of Downton Abbey take on the roaring 1920s

"Shirley's a Hollywood legend and Maggie's a theatre and film legend, [so] for them to meet was great," says Carter. "It was just lovely to be around.

"And we got her to sing If They Could See Me Now [from Sweet Charity] for us, which was a treat."

Hollywood also seems to have rubbed off on producers' efforts to keep forthcoming plot details secret, with cast members revealing scripts have to be "shredded and burned" once memorised in true blockbuster style.

Yet Elizabeth McGovern, who plays Countess Cora, reveals both new faces and the glamorous 1920s will add to the drama.

"There's a very tall footman, a very beautiful second cousin [and] a very fetching new downstairs maid who sets the boys' hearts spinning.

"And it's amazing to see the girls come to life when they're free of their corsets," the actress adds.

"They have oxygen pumping to their brain and they get up to a lot more, I don't think it's a coincidence."

As for the future of Downton, there are more rumours Dame Maggie is ready to call time on her role. But Fellowes insists nothing is confirmed.

"You have to judge it when you've clearly peaked and try and end it before you go down the other side, but I think there is another series in it," he says.

"ITV keep their cards pretty close to their chest. Someone was saying, 'Oh, so and so hasn't been signed.' Nobody's been signed! We are not yet commissioned to make series four.

"I think I'd be surprised if we weren't asked, particularly because of all the Emmys. But we haven't been yet."

Downton Abbey begins on ITV1 on 16 September at 21:00 BST.