Fallen angel: How Lily James is bringing scandal to Downton Abbey this Christmas

She might have been banished from the Abbey, but that won’t stop Lily James from wreaking havoc on Downton’s Christmas. Gerard Gilbert meets the vivacious young actress whipping Britain’s favourite period drama into a frenzy.

After her first day of filming Downton Abbey at Highclere Castle in Berkshire earlier this year, the 23-year-old Lily James had a glimpse of what a charmed circle she had just joined. Returning to the local hostelry favoured by the show's actors, the Carnarvon Arms Hotel (the current Earl Carnarvon owns Highclere), she discovered "the whole cast playing croquet and drinking cider outside this picturesque little hotel on a sunny afternoon in the country. I was like, 'Are we working or is this just a dreamy holiday?'"

Obviously there was hard labour to come, including the daily application of a curly, strawberry-blonde wig to disguise her naturally straight brunette hair, which had been deemed too similar to fellow younger female cast members Laura Carmichael and Michelle Dockery.

James's character, Lady Rose, the wild-child great-niece of Maggie Smith's Downton dowager, made her dramatic entry in the most recent episode of the globally popular costume drama (it is shown in more than 100 countries), breaking away from her chaperones, Matthew and Lady Edith, to consort with a married man in a louche Soho jazz club – a jolly-looking place likened by Matthew (Dan Stevens) to "the outer circle of Dante's Inferno".

The upcoming Christmas special sees the Crawley family upping sticks from Downton to visit Lady Rose's ancestral pile in Scotland (actually Inveraray Castle in Argyll), whither the fun-loving flapper has been banished since her misdeeds in the metropolis. "I'm in the middle of nowhere with my awful mother [played by Phoebe Nicholls] and just desperate for some life and some, um…" Married men? "Some married… well… I'll keep schtum about that… no… she's just like a trapped bird."

Similarly under lock and key are the exact details of the Christmas storyline, journalists only being allowed to preview the two-hour episode tomorrow, on Christmas Eve. "We were all waiting on tenterhooks for this script and I can honestly say I couldn't stop reading it," says James, who has been "optioned" for the fourth series, which begins filming in February. "I'm so excited for people to see it."

As any late-coming Big Brother or I'm a Celebrity contestant will tell you, joining an established group can be tricky, added to which the three actresses who play the Crawley sisters – Carmichael, Dockery and the now-departed Jessica Brown Findlay (who played Lady Sybil, RIP) – are a famously tight posse, socialising together in London when not filming. "It's a very settled cast and everyone's really close," says James, "and obviously my character comes in after Sybil dies and they're all missing Jess, but actually they were all lovely and welcoming. What was terrifying is that the show is such a success and you just don't want to let the side down."

She has hardly done that, although Lady Rose can, admittedly, be on the annoying side. "She's quite k petulant and young – probably me when I was 16," says James. "Julian [Fellowes] writes as it goes along so when I auditioned he hadn't actually written Lady Rose yet except for, 'She's a wild cousin from London who comes to the country to be tamed' – that's all I got. In one of my first jobs I played Ethel Brown, the older sister in Just William… there were similarities there, I thought. She's also been seduced by that underworld of jazz and drinking, but what I really like is that she's just bursting with life."

The same can be said for James, an alert, youthfully vivacious presence given to anticipating the ends of my questions, and following her answers with a cheery chuckle. One of her reviews for the recent Southwark Playhouse production of Chekhov's The Seagull mentions "Lily James's captivatingly fragile teenage Nina, who, from her first entrance, never seems to know what to do with her hands." James's digits are similarly restless now, either playing with her hair or tugging at the sleeves of her stripey top. "Twenty-year-old Marks & Spencer," she jests. "I love clothes but it's a struggle for me to get out of jeans and a baggy jumper. It was really nice for me to play Lady Rose and be wearing lots of pink; I'm very rarely out of black or brown."

Since leaving drama school at London's Guildhall in 2010 James has played Desdemona at the Crucible, Sheffield, alongside The Wire co-stars Clarke Peters and Dominic West's respective Othello and Iago, as well as appearing in the acclaimed Young Vic revival of Vernon God Little. On television there was the aforementioned Just William and the role of Poppy, Billie Piper's flatmate in the final series of Secret Diary of a Call Girl.

"She wasn't a call girl but she did have her slightly risqué moments, which weren't great for my family," says James. "Although Lady Rose is pretty naughty there's definitely no nudity so I'm very glad that my granny can be proud of me in this, and my brothers don't have to turn away." Her brothers are Sam, a student, and Charlie, who works for Sky Sports and "came to see me in The Seagull five times… he's very supportive".

Her Secret Diary co-star Billie Piper is married to Laurence Fox, with whose cousin, Freddie Fox, James was at Guildhall ("We were born on the same day… he's my wonderful friend"), and whose younger brother, Jack Fox (JP's obnoxious public-school friend, Ralph, in Fresh Meat) has been linked romantically with James. "This is rumour," she says. Can she confirm it? "He's a er, er, er, I, I, I, we, we, we, yeah… he's a good friend of mine." Her publicist later e-mails to clarify that they are not in an amorous relationship. Either way, we agree there are a lot of Foxes about. "You can't really escape the Foxes; you'll meet one of them somewhere."

But then James herself is not without thespian pedigree. Her American-born grandmother, Helen Horton, appeared in countless films and TV shows including being the voice of Mother, the ship computer, in Ridley Scott's Alien. "Which is pretty cool, I think," she says. "When I tell friends who are into sci-fi they're very impressed."

But it's from her late father, James, that she takes her stage name. Born Lily Thomson ("without a 'p'… the bane of my life"), Thomson/James was born in Esher, Surrey, in April 1989. "My dad did all sorts of things," she says. "He was an actor for a while, in fact I've just found these old head shots of his in the attic. He was also in a band with Tony Head [Buffy star Anthony Head] called Two Way All Dressed Up. I've got this really cool old record…"

James inherited the performance gene from this side of the family ("Mum would have a panic attack if she had to stand up and give a speech around a table"), attending the £21,000-a-year Arts Educational School in Tring, Hertfordshire – alma mater of Julie Andrews, Sarah Brightman, Jane Seymour as well as current and former Downton co-stars Amy Nuttall and Jessica Brown Findlay. "Because it's a performing arts school sometimes I go, 'Oh God, I shouldn't mention I went there,' because you seem like you're just some sort of cattle-market child who wants to be famous," she says. And I must admit that after reading James's CV, I had been half-expecting a self-obsessed, robotically ambitious stage-school graduate; it's always pleasant to have one's worst suspicions confounded. "Boarding school in Tring was a bit of a bubble," she continues. "That burst when I went to Hackney to go to drama school."

James still lives in Hackney – in Homerton, to be precise – sharing a house with "two guys I went to school with… sort of actors, sort of DJs, sort of barmen", although she has been away notching up her first blockbuster, a mash-up of Greek mythology called Wrath of the Titans, for which she spent six weeks in Tenerife with Liam Neeson, Rosamund Pike, Ralph Fiennes and Bill Nighy. The 3-D epic passed me by, I admit. "I wouldn't worry too much," she laughs. "I did enjoy it – just for the experience. I got a real taste of what it's like to make big studio movies. I was in the middle of the desert filming with these huge cranes and bombs going off and thinking, 'How lucky am I?'"

Then there was this summer's British movie Fast Girls, a tale of rivalry between female Olympic hopefuls, the working-class Shania (Lenora Crichlow) and the middle-class Lisa (James), that had (largely unfulfilled) aspirations to be an athletics equivalent of Bend it Like Beckham. James trained five times a week with Team GB, running up hills until she vomited and clocking up 88 sit-ups a day until she had washboard abs. "No washboard stomach now," she says, patting her belly. "It's a distant memory." As is the filming, which she admits was "an unhappy experience. It was just hell. I became so obsessed with training and I've got an addictive personality, although it was really exciting to be training at Lee Valley. Dwaine Chambers was training there, and the 100-metre racers – and I did a whole sit-up session with them. It was just mad, yeah."

Such a role has inevitably led to James being dubbed the next Keira Knightley – and there is a certain physical likeness, although James is curvier (how could she not be?). "The next Keira? I'll take that as a nice thing because just as many nasty things are written." Really? "When I worked with Bill Nighy on Wrath of the Titans, he said to me, 'There's one thing you can promise me and that's never ever, ever, read your reviews.' And I have taken that on board with the theatre, but for some reason I looked at stuff with the television I've done and it's silly; I won't again."

James has an American agent, and in January will be joining the annual mass exodus of young British talent to Hollywood for the TV pilot season. "The famous pilot season literally sends shivers down my spine," she says. "I've heard such horrors of how many actors and actresses are out there. My dad lived on Sunset Boulevard for a couple of years as a waiter and he said he'd do a different character every time somebody sat down, just to get some practice. I really want to play interesting roles, but you want to work, so it's a balancing game."

Certainly her choice of role models is sound, and James is drawn to actresses who, like herself, mix stage and screen. "I think it makes you a stronger actor," she says. "I just saw Cate Blanchett at the Barbican, and to watch her on stage… she's just magnetic. And even Billie Piper doing The Effect at the National, and Sheridan Smith and Hayley Atwell. And Felicity Jones because she's done Chalet Girl, which is a chick-flicky movie, but she also did Like Crazy."

Longevity is, of course, the name of the game for anyone serious about an acting career. How was it performing with Dame Maggie Smith, her great-aunt in Downton Abbey? "When Freddie Fox found out I had the job he sent me California Suite [Neil Simon's 1978 comedy with Smith and Jane Fonda] and I was like, 'Thanks Freddie for making me even more nervous about meeting her.' She's just so fierce and full of life. And she's so funny as well. I was in one of my blue-and-white, slightly awkward dresses and she said that I looked like a toilet-cover case. It was very true."

'Downton Abbey' is on ITV1 on Christmas Day