For a series with regeneration at its core, Doctor Who can struggle with periods of change. The biggest year in the show’s life began with a behind-the-scenes drama surrounding the sudden departure of executive producer Caroline Skinner.

Now, as Jenna-Louise Coleman makes her third ‘debut’ appearance, as new companion Clara Oswald, there is a danger that her first full series will be eclipsed by the excitement around November’s 50th anniversary episode.

Clara, over the course of eight new episodes, joins the Doctor in battle against revamped Cybermen, updated Ice Warriors – back on our screens after 38 years – and even a power-dressing Celia Imrie, as well as exploring more of the Tardis than we’ve ever seen before as we gradually uncover her true identity.

Observing Coleman on-set at the Grange Hotel in St Paul’s, London, during filming of the series opener, The Bells of Saint John, she doesn’t appear remotely daunted at the prospect of following in the wake of the popular Amy Pond. Coleman, whether collapsing into fits of laughter at Matt Smith’s super-expressive chin (causing a short delay in filming) or reeling off her alter ego’s quickfire dialogue, has taken to the crazy world of Doctor Who like a Dalek to slaughter.

But it’s not surprising to find many eyes fixed on the show’s birthday celebration. For, not only is the anniversary episode a feature-length special – filmed in 3D for the first time, and simulcast in cinemas across the country – it is also likely to feature former Doctors David Tennant, Christopher Eccleston and, yes, even Paul McGann alongside the present incumbent Matt Smith.

“It manages to pay homage to everything and look forward – I think that is the genius of it,” is Smith’s cryptic verdict on Steven Moffat’s anniversary script, which he finally received earlier this month.

The way this series ends is mindblowing, especially for long-time Whovians

“He has written one of his vastest, most epic, funniest… I mean, he is on top form. And when he’s on that form, it’s completely, undeniably brilliant. I read it and clapped at the end. I just loved it – there are belting moments to get your inner fanboy really tingling.”

Coleman agrees. “It is massive!” she says. “The way this series ends is mindblowing, especially for long-time Whovians. It is going to be really special and feeds into the 50th anniversary episode. It’s a treat for fans of the last 50 years.”

Tennant and Eccleston deny they have heard anything about the anniversary episode. But then a coaching manual on obfuscation techniques accompanies the casting of any actor in a lead role in Doctor Who. “It is a new skill for me, saying a lot without giving anything away,” admits Coleman. “You find yourself apologising for being totally vague and a big tease, but there is not a lot we are allowed to say.”

Still, we are promised that the Doctor’s greatest secret will be revealed ahead of the anniversary episode. “It actually will, I’m not lying,” says Moffat. River Song (Alex Kingston), who knows all the ‘spoilers’, appears in the series finale, guiding the Doctor towards what will become the most talked about 90 minutes of television this year.

But should we be looking so far ahead? Should we be mentally fast-forwarding through the latest series, just as we did in the 15 months between Smith’s announcement as Tennant’s successor and his first full episode? Well, no. Because we have a new companion to meet, and Moffat is the master of the introduction.

Millions marvelled as he won over sceptics with his expertly crafted first episode of Sherlock in 2010. Smith’s debut in The Eleventh Hour – he had me at ‘fish custard’ – saw the writer weave a lifelong friendship between Amelia Pond and her raggedy man, adding scares, outfit choices (“bow ties are cool”), and a brief-but-charming tribute to his Time Lord predecessors in one family friendly, sci-fi entertainment package.

Now he’s at it again with new companion Clara (when pushed for a preference, Coleman, disagreeing with me, decides “companion” sounds marginally more equal, more active, than “assistant”).

If the hangover from Billie Piper’s heartbreaking exit as Rose Tyler overshadowed Freema Agyeman’s single 2007 series as Martha Jones – “I felt it affected Martha’s journey quite a lot that he was always talking about Rose,” says Smith – it’s no problem this time around.

With almost indecent haste following the trapped-in-New-York-forever “demise” of Amy and Rory, Smith’s Doctor has a new obsession. And this is far more than an intergalactic, timey-wimey rebound romance. The signs, already apparent from Coleman’s shock appearance in Asylum of the Dalek last year and the finest Christmas episode since the series returned in 2005, are that this is the beginning of another beautiful friendship. And maybe more…

“It is like they love each other and can’t stop hanging out with each other, but just can’t figure each other out,” is Coleman’s take on the new dynamic. “So they are staring at each other, with all these mathematical equations going through their heads at the same time.

“It changes the Doctor so much because there is nothing he hates more than an unsolved equation. I don’t think they have ever done this kind of mystery about the companion before. It is something totally new.”

Even in her third ‘first episode’, Clara’s true identity will not be revealed – although we can safely assume that she is neither Dalek nor, judging by the computer skills she displays, Victorian barmaid.

Whereas Karen Gillan – not unlike Amy Pond – is all flailing limbs and gawky enthusiasm, Coleman already appears more self-assured.

“It’s funny,” says the 26-year-old Coleman, who made her name as Jasmine Thomas on Emmerdale, and since leaving the soap in 2009 has chosen projects wisely. She has appeared in Stephen Poliakoff’s Dancing on the Edge and the remake of Room at the Top, as well as the less impressive Titanic.

“Since filming finished, I’ve been sitting in the pub catching up with my friends and talking about what we have all been up to. I just keep having flashes in my head of me with a mace attacking a Cyberman. I do feel like I’m living quite a different existence at the moment. But I have absolutely fallen in love with Clara.

“It is telling of what is to come in that she stands on her own two feet, she is very resourceful. The Doctor is used to being the clever one, but every time the Doctor could fix something with his sonic screwdriver, Clara pulls a hammer out of her bag that will do the trick.”

The Bells of Saint John is set in the not-altogether alien territory of modern-day London. Against a backdrop of the city’s picture postcard greatest hits – Big Ben, the Shard, the Houses of Parliament – it’s more an off-kilter action movie than monster raving sci-fi knockabout.

Smith doesn’t waste a single frame, his performance so inventive, so energetic. That Coleman holds her own – and, whisper it, can talk even faster than her co-star – is hugely impressive.

“This episode is our take on James Bond,” she says. “There was a scene on the motorbike, which I totally felt was going to be my Bond Girl moment – on the back of the bike with the wind flowing through my hair.

“But then you turn up and see the costume – these geeky goggles and a Mr Bean-style hat – and I’m too short, so couldn’t see over Matt’s shoulder. So you realise, yes, we are filming Doctor Who. You can’t try to be too cool with it…”

Doctor Who, BBC One, Saturdays, 6.15pm

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