San Diego's Comic-Con festival, held each July, is the densest concentration of nerds in our galaxy. For the duration, grown men and women walking around in superhero costumes is the norm, not the exception. Earlier this year, Jenna Coleman – the 27-year-old actor formerly known as Jenna-Louise Coleman (only her mum still calls her Jenna-Louise apparently) – went to her first Comic-Con. There were 130,000-plus attendees; tickets had sold out in 93 minutes. Along with Matt Smith, her co-star in Doctor Who, she spent four days being spirited through hotel kitchens, out of back doors and into cars with forbiddingly opaque windows.
Not that Coleman and Smith remained incognito for long. "Nice costumes!" they screamed out of the car window at one middle-aged couple dressed as the Doctor and Clara, their characters from the series. The man didn't recognise them, but "Clara" did, and appeared to start convulsing on the pavement. "The most embarrassing thing is that the traffic is so bad that you don't go anywhere," says Coleman. "So all you can do is sit there and put the window up."
Comic-Con was Coleman's first proper exposure to the fanaticism of the Whovians. She had never watched Doctor Who before she became the new "companion", but the responses to her performances have been effusive, bordering on obsessive. Doctor Who blogs – of which there are legion – praise her as quick-witted and independent yet vulnerable, and are particularly taken with the flirtatious relationship she has with Smith's Doctor – a spark that was absent with his previous companion Amy Pond, played by Karen Gillan. Or, as Matt Smith himself put it: "Clara's different from Amy. He has more chance of snogging Clara."
While Coleman knew Doctor Who inspired extreme passions, it had not really hit home until Comic-Con. "I was always asked how I had found the fans, but I'd just been filming in Cardiff," she says. "At Comic-Con it was amazing to see how far-reaching it is. I thought I'd be overwhelmed, but I was humbled. It's something that Matt says: the star is the show."
That maxim is more obvious than ever this year as Doctor Who celebrates its 50th birthday. The centrepiece is a 75-minute special on 23 November called the Day of the Doctor, which was shot in 3D and will be shown on BBC1 and in 400 cinemas across eight countries. The episode will bring together Smith and Coleman with some of their predecessors, including David Tennant and Billie Piper, and introduce a new "dark" Doctor, John Hurt.
After that, Smith will be hanging up his bow tie and vintage Harris Tweed jacket in the Christmas special. The speculation over his successor, which shared a hysteria in common with the announcement of a new pope, ended in August when Peter Capaldi was unveiled on primetime television as the new pontiff – sorry, 12th Doctor. Coleman only found out herself a short time before the rest of us.
"They told me and Matt when Prince Charles and Camilla came to the set," says Coleman. "We were both: 'Ahhh, of course.' It takes you a few moments – I don't think he was on any of the original lists. People were talking about Rory Kinnear and people like that, but as soon as you say it, you're like: 'Of course.' As Steven Moffat [Doctor Who's lead writer] said: 'He's the Doctor.' And it's brilliant that we've gone so different from Matt."
Smith's final appearance, however, will clearly be a wrench. "I just read the script the other night," says Coleman. "I'd been putting it off for ages and ages, because once you read the last page, that's it, the story is over. So I read 10 pages on the tube and I stopped, and then I picked it up again the other day and finished it. I was an absolute mess, an absolute wreck. But it's good; it's sad, but it's what needs to happen. It's perfect."
Everything is looking good for Coleman right now, but, over a coffee in an east London café, there is a wariness as she talks about her career. It is not so long, after all, since she was unable to book an audition – for anything. She worked in a bar and attempted to get into Rada, but froze in her admission interview, forgot all her lines and was turned away. "I'd always wanted to be an actress," she admits. "I was like: 'What if I've been wrong all along?'"
Coleman does not come from a long line of performers. She was born in Blackpool ("a great place for a Doctor Who episode: it's weird, quite romantic, but it's not found what it's supposed to be now") and her dad – who has a business, with her brother, fitting the interiors of bars and shops – would watch her in school productions and wonder where the acting bug had come from. Aged 11, Coleman appeared as a bridesmaid in the musical Summer Holiday with Darren Day, and the singer gave her a Debenhams voucher as a thank you.
She had some wild moments in her teens – "I was a bit rebellious from 14 to 17, if you know what I mean" – but pulled it round to become head girl at Arnold School and get good grades. Coleman had a place at York University to study English literature, but was offered a spot on Emmerdale and took that instead. It was great – lots of acting experience, decent pay, living in Leeds – until her storylines dried up. "I had about six months where I wasn't doing very much on a day-to-day basis, just going into the pub and sat having a chat," she recalls. "So that's when I decided to leave, and that's when I ended up getting storylines."
Coleman's not kidding. In short order, her character Jasmine Thomas had a lesbian affair with her best friend Debbie and became pregnant by her father, Cain Dingle. She had an abortion and another fling with a local copper, Shane Doyle, before clubbing him to death with a chair leg and being sent to the slammer.
But after almost four years on the soap, Coleman couldn't get another job. She moved to London, took some bar shifts and started an Open University degree in English before deciding to try her luck in Los Angeles. There she rented a room off an old lady in West Hollywood and went to auditions most days.
"I was going for parts I was never in a million years going to get," she says. "Like a 30-year-old wife, and at this point I looked so young – not that I look much older now. But it wasn't about that. I just relished walking into an audition room with people with an open mind and getting to read. I must have had 40-odd auditions in three months – it was relentless, but I came back to England a lot more fearless."
Coleman did get one part: the "tiniest, tiniest thing" in the 2011 tights-and-fights action film Captain America: The First Avenger. But that was enough. It led to a bigger role in the BBC4 adaptation of John Braine's novel Room at the Top, which led to Julian Fellowes casting her in Titanic, which led to Stephen Poliakoff choosing her for Dancing on the Edge, which aired on BBC2 this year.
Any struggles certainly seem long distant now. Next month Coleman will appear in the BBC three-parter Death Comes to Pemberley, PD James's smart, what-happened-next take on Jane Austen's Pride and Prejudice. The action starts six years on and Coleman plays Lydia Wickham (née Bennet), Elizabeth's younger and perennially self-absorbed sister.
"Lydia's basically very hysterical, and I've had a lot of licence to go wild with it," Coleman says. "I went through the book and I wrote down all the words she's described as and it's like: ignorant, idle, wild, volatile, indulgent. The director was like: 'We should want to slap you in the face.'"
Between Doctor Who and Death Comes to Pemberley, time travel and period drama, Coleman says she has had a hectic few months. She has barely been home and hardly seen her boyfriend, Richard Madden, who has been quite busy himself as Robb Stark in Game of Thrones. "We're both young and I want us both to have our adventures and do our thing," she says. "But if you want something to work, it'll work."
In fact, the problem, Coleman finds, is the real world becomes rather dull when you are not slaying Cybermen all day. "Doing Doctor Who you're on a cloud, doing stunts, being dropped in gloop," she says. "Then suddenly you stop and I'll be walking round thinking: 'Real life is actually a bit boring.'"