Doctor Who star Jenna Coleman: "Peter Capaldi is almost the opposite of Matt Smith"

As she returns to the Tardis for a second series, the actress discusses Capaldi versus Smith, alien chit-chat… and the advantages of being 5ft 2in

Doctor Who has launched Coleman towards a global audience (Benjamin McMahon)

It's approaching seven o'clock on a Friday evening in a forest near Cardiff and filming of the new series of Doctor Who has wrapped for another week. Jenna Coleman, soon to complete her second series as the Doctor's latest companion, Clara Oswald, prepares to be driven back along the M4 towards London for a precious two-day reunion with her boyfriend, the Game of Thrones actor Richard Madden.

"This is how we spend our Friday nights, travelling from Cardiff to London," she says, "we" apparently referring to gaggles of commuting TV performers. "There's so much being filmed in Wales at the moment – Doctor Who, Da Vinci's Demons, Atlantis – that I keep bumping into actors at the service station."

Series eight of the rebooted time-travel saga began filming at the Cardiff Bay studios of BBC Wales back in January, and only completed earlier this month. "The job is so consuming," says Coleman. "When I'm here in Cardiff, it's me and Peter, all day, every day."

Peter, of course, is the 56-year-old Scottish actor Peter Capaldi, formerly best-known as the sweary Thick of It spin-doctor Malcolm Tucker, now reincarnated as the 12th Doctor. Capaldi has revealed that, unlike with his predecessor (played by 31-year-old Matt Smith), whose Doctor was engaged in a close relationship with Clara that once led to a passionate kiss, there will be no flirting between the Time Lord and his companion here.

"I always thought Matt was so young- looking but had this older, wiser quality about him, whereas Peter is almost the opposite," observes the 28-year-old Coleman. "Somehow he has this energy that is younger. Visually, obviously, it is very different."

Despite rumours ranging from the plausible (Rory Kinnear) to the bizarre (David Beckham), Capaldi was the one and only choice of showrunners Steven Moffat and Mark Gatiss to replace Smith, the news being revealed to Coleman on the same day that Prince Charles and Camilla happened to be visiting the set.

"They told me and Matt together," she says. "I don't think even Peter had any idea that he was the only person who'd read for the part."

What was it like, adjusting to a new actor in the role? "He looks completely different and acts completely differently," says Coleman. "We're discovering that it's much more of a turbulent relationship [between the characters]; he brings out the control freak in Clara because she can't quite pin him down. It's always an interesting dynamic with the Doctor, anyway; one moment he's your friend, and in another moment he's this weird alien, and in another moment he's being this annoying kind of toddler and you're the adult, and in the next moment he's playing the wise old grandfather."

Coleman is herself relatively new to the show, having gradually replaced Karen Gillan, who played Amy Pond, during the course of the seventh season, Moffat claiming that one reason he cast her was that she was the only potential companion who could speak faster than Smith. "That comes in handy with Doctor Who: there's so much story to tell in 45 minutes."

"The auditioning process was really unusual," reveals Coleman. "Steven and the other producers and the director all left the room and left me and Matt for about 50 minutes to chat through the scenes and play off each other. It was very relaxed. I had no idea what they wanted at all, I was just playing around with accents and going from Mary Poppins to this London girl to this person from a different planet, while Matt kept throwing different curve balls at me."

Before she auditioned, Coleman watched the 2010 Matt Smith/Karen Gillen episode "The Eleventh Hour", because she had never seen the show before – unsurprisingly, since she was only four in 1989, when the BBC pulled its original incarnation of Doctor Who. "And I was about 18 when it came back with Billie [Piper] and Chris [Eccleston]. It was something that I was very aware of because it was kind of everywhere, but it simply wasn't on during my childhood."

That childhood was spent in Blackpool, a town Coleman thinks would make a brilliant setting for a Doctor Who story. "You can imagine all those lights and all the amusements turning against you somehow." Her father and brother run a shop-fitting business, and Coleman has no idea where her theatrical urge came from. "It's something I've been doing from a really young age but it's not in the family at all."

Aged 11, she appeared as a bridesmaid in the musical Summer Holiday with Darren Day, receiving a Debenhams voucher for her efforts. After a rebellious patch in her mid-teens, she went on to become head girl at Arnold School in her home town, and won a place to study English at York University, although before she could take it up, she was offered the part of Jasmine Thomas in Emmerdale.

Jasmine's storylines eventually started drying up, so Coleman decided to leave – and things got decidedly racier: Jasmine had a lesbian affair with her best friend Debbie, became pregnant by Debbie's father and had an abortion before clubbing her policeman boyfriend to death with a chair leg. "A really good exit story," she calls it.

Like so many soap actors before her, Coleman discovered her stint on Emmerdale had categorised her in the minds of casting directors: "It was hard for people to see you in a different role." So, having moved to London, taken some bar shifts and started an Open University degree in English, Coleman decided to try her luck thousands of miles from such preconceptions – in the annual Los Angeles TV pilot season, renting a room in West Hollywood.

New dimension: Coleman in Doctor Who with Matt Smith (BBC)

"It was great because I never had a gap year or anything like that. You'd get a pilot that was set in a 1930s jazz bar and you'd go off and just look into that for a day, even if it wasn't a part you'd get in a million years; I enjoyed the process and just reading different scripts. I came back to England a lot more fearless." And also with a tiny part in the 2011 action movie Captain America: The First Avenger – enough to snag a leading role in the BBC4 adaptation of John Braine's Room at the Top, before she was cast in Julian Fellowes' Titanic and as Rosie in Stephen Poliakoff's Dancing on the Edge. "I've never worked with a director who'd written something before, so it was really interesting," she says of Poliakoff. "The detail in everything was immense, how the camera was going to pan… you could really feel how it was going to work."

Doctor Who has launched her into a completely new dimension, however – towards a global audience, some of whom she will meet as she joins Capaldi on a promotional world tour taking in South Korea, Sydney, New York, Mexico and Rio de Janeiro. "It's going to be a proper shock to the system," she says. "We've been living in this mythical world for seven months – chasing things and green screens and aliens – and now we're going to go out into the real world."

Then there are the Whovians: how does an actor relatively new to the whole Time Lord dynasty cope with fans steeped in 50 years of Doctor Who lore? "I've swotted up on it," she admits. "There's so much mythology and some important stuff you really do need to know. For instance, there was something in the episode we were doing yesterday that we didn't understand so Peter Googled it and it went back to 40 years ago and one of Jon Pertwee's episodes."

Coleman says: 'There's so much mythology ... There was something in one episode we didn't understand. Peter Googled it, and it went back 40 years to a Jon Pertwee episode' (Benjamin McMahon)

Fans are usually respectful and keen only to discuss the finer points of the show, and she hasn't met anyone seriously creepy yet, claiming she's still able to ride undisturbed on the London underground. It also helps being 5ft 2in: "I walked past two people with Doctor Who shirts on the other day who completely didn't see me."

In between her first two series as Clara, Coleman, who had been billed throughout her career as Jenna-Louise Coleman, suddenly lost the Louise bit – prompting one newspaper to report on "the mystery of the missing Louise". "Peter now has a nickname for me: 'the artist formerly known as JLC'," she says. "The boring truth is that people have never really called me Jenna-Louise and I just found it very strange because I started to do more interviews and go places where people I didn't know kept calling me Jenna-Louise. It sounded odd to me."

After a rough cut of the opening episode of the new series was leaked online last month, along with scripts for a further five episodes, Coleman is understandably vigilant about not dropping further storyline details – though she will reveal that her oddest moment was fighting with a man while dressed as a Victorian governess. "And you keep having these out-of-body experiences, like finding yourself chatting to aliens."

Coleman in the new Doctor Who series with Peter Capaldi (PA)

How weird is it to meet your first Dalek? "Ha! I turned into a Dalek in my very first episode. That's way more cool than just meeting one."

The only advice Steven Moffat gave Coleman was to react to what was going on around her. Did Gillan pass on any tips? "My first day we were in the make-up chairs together as I was in one of Karen's episodes. She was really supportive and sent me messages saying, 'Just enjoy it,' and gave me tips on Cardiff. I wish I had asked her advice as there are so many questions you want to ask. But it's the same as has happened with me and Peter – it kind of feels like a new show in a way."

Billie Piper aside, who was already a teenage pop star and married to Chris Evans before her stint as Rose Tyler, being Doctor Who's female companion has been a career pinnacle for most of the actresses involved. From Elisabeth Sladen (Jon Pertwee and Tom Baker's companion, Sarah Jane Smith) and Katy Manning (Pertwee's longest-serving partner, Jo Grant) to more recent manifestations such as Freema Agyeman (David Tennant's sidekick Martha Jones) and Gillan – whose upcoming US sitcom Selfie is reportedly pretty dire – the BBC sci-fi classic has thrown a very long shadow indeed on its young female co-stars. It's not, however, something Coleman is thinking about right now.

"At the moment I'm really happy," she says. "It's one of those jobs that will only happen once – the whole adventure that it brings. The key is not to worry about the future, and enjoy it."

'Doctor Who' returns to BBC1 on Saturday