The courtyard of the BBC studios in Cardiff is littered with bodies. Not the sort of extinguished alien lifeforms you might expect to see at the home of Doctor Who. Instead, they're scrubs-clad Casualty actors, lunch-breaking and lounging in the mid-June sun, overlooked by the cheerily vacant streets of long-running Welsh soap Pobol y Cwm. Even as Peter Capaldi – AKA Time Lord #12 – starts weaving his way through pretend medical staff, it still doesn't feel much like the home of a culturally momentous sci-fi franchise. In fact, the grey-blue hangar a couple of hundred metres down the road that houses the Doctor Who Experience – which on my way here I dismissed as a hollow money-spinner – is starting to look positively atmospheric.

Admittedly, this sort of flagrant mundanity isn't totally out of keeping with the show itself. Part of Doctor Who's attraction lies in its imaginative potential: it's held a sense of wonder, awe and terror for generations - not bad for a show whose most enduring enemy resembles a slow-moving bin. "It is this relationship between the domestic and the epic," says Capaldi of what appeals to him about the programme. "The sense that there's a bridge, that a hand can be extended, and you can step from the Earth, from the supermarket car park, into the Andromeda nebulae or whatever." He pauses for a second. "And I love monsters. Everybody loves monsters."

These days, Doctor Who is big business, reaching way beyond its real-life surroundings. Distributed to more than 200 territories worldwide, it's a global brand, and ranks as BBC America's highest-rated show. It's also relentlessly chewed over by the media. Few things better demonstrate the newsification of pop culture than the slow dissemination of Who-based information we've had over recent months: trailers teased within an inch of their lives; security-breach stories unfolding over days; every cast and crew soundbite on the new series mulled over as if the future of the human race really did depend on it. Who news is entertainment in itself: last August, the announcement about the casting of the 12th Doctor was spun into a TV special, hosted by Zoë Ball and watched by more than 6 million viewers. Capaldi's identity was such a valuable and anticipated secret that he admits he was chauffeured around pre-broadcast with a blanket over his head.

Capaldi's Doctor in pensive mode. Photograph: Adrian Rogers/BBC/Adrian Rogers

For someone as established and respected as Capaldi, you get the feeling this kind of build-up – and the potential for freefall – is not ideal. On approach, he looks a bit like the ageing rock star he might have been (he was famously in punk band the Dreamboys with US chatshow host Craig Ferguson in his youth), wiry in dark glasses and heavy boots. But once he sits down, his hands, which he's contorting anxiously around the slots in the tabletop, give away at least some discomfort.

If he's nervous, that's unsurprising. The pressure doesn't just come from being the face of a powerful franchise; for someone like Capaldi, becoming the Doctor has added layers of significance. Like David Tennant before him, and showrunner Steven Moffat (who wrote Doctor Who fan fiction), it's well known that Capaldi was once Doctor Who mad. As a teenager he inundated the production team with fan mail. In the live TV announcement, he was presented with a letter his 15-year-old self wrote to the Radio Times praising its Doctor Who coverage, which Capaldi sheepishly referred to as "the full anorak". Even today during our conversation there are moments when he seems more like the trivia-addled president of a Doctor Who fan club than the actual Doctor himself, talking about the red-lined coat and Pertwee-esque pose in the promotional pictures, and the token William Hartnell lapel-hold he did on the live show ("They applauded for too long. I thought, 'I can't stand here!'"). Reminiscing about surreal on-set moments, he remembers the first few times he filmed in the Tardis. "I had to be very patient," he says, with a hint of nerdish derision, "because there were always very nice prop guys telling me how to work the Tardis, and I was like: 'I know how to work the Tardis! I've known for a very long time how to work the Tardis. Probably longer than you. So you don't need to tell me!'"

That a significant portion of the programme's 21st-century regeneration has been created by Doctor Who fanatics is fitting. Moffat has made it increasingly self-referential (the opening episode, Deep Breath, is littered with allusions to both age and Scottishness), but the show's detailed, complex and eccentric universe has always lent itself to fandom. For many, there's also a strong emotional attachment to the programme that stems from childhood. It's a nostalgia Capaldi himself embraces, putting it in a bracket with "the Beatles and Sunday Night At The London Palladium, and school milk and bronchitis, and smog and little S-buckled belts". However, there is a risk that heritage can also end up as baggage; reboot blurring into tribute. "I thought: 'This is going to be like Stars In Their Eyes,'" he remembers of the live show. "Tonight, Matthew, I'm going to be… Doctor Who!"

Although there's little danger of Capaldi resembling a middle-aged man living out his teenage delusions amid a cloud of dry ice, the new Doctor is still going to need to commandeer the part for himself. His casting marks a departure from Tennant and Matt Smith's bouncy young Doctors, which might be a risk considering Doctor Who relies on new generations of devotion. That aside, for many people just knowing such a charismatic actor will be playing the Doctor is enough to make the show an exciting prospect again. In some eyes, the larger story arcs of the past few series were running out of steam, but now the majority of loose strands – including the mystery surrounding companion Clara's multiple selves – seem to have been tied up. Combined with the assembly of past and present Doctors in the recent 50th-anniversary episode, there's an added sense of rebirth about the new series.

Capaldi himself is conscientiously vague about where he'll be taking his Doctor, saying only that he "struggles through all the wreckage to find himself". In light of the opening episode, that seems like an understatement. Freshly regenerated, his Doctor ambles round Victorian London wild-eyed in a nightshirt, insulting people while struggling to make sense of himself and his surroundings, like a sort of wise-cracking King Lear. There are moments of comedy, but there are some unsettling undertones if you choose to look for them. On the Doctor's other characteristics, though, Capaldi is evasive, admitting only that he's "a slightly more mysterious figure".

Coleman and Capaldi in a scene from the top secret new series. Photograph: Adrian Rogers/BBC/Adrian Rogers

If he is overly careful not to impinge on his Doctor's mystique by doing something like answer a question, that's understandable, there being so much riding on this new incarnation. But perhaps there'll be some clues on-set. In a small studio a few corridors away, director Douglas Mackinnon is filming a sequence in which a chintzy sofa gets crushed out of existence. This will be the ninth episode, in which Jenna Coleman's Clara must lug the Doctor and his Tardis around in her handbag after they get shrunken down to miniature size. As proof, two tiny Tardises stand side by side on a nearby table.

With such living room-centric filming going on, it's decided by executive producer Brian Minchin that I should at least get to see the Tardis proper. A cry goes up among the crew to switch on its lights but by the time we arrive we're greeted by a wall of darkness. By the light of an iPhone screen I take a few tentative steps into its shadowy womblike interior, vaguely able to make out the upper storey and central console in the gloom. Just as I start feeling a lot like a wide-eyed companion – one that probably has an alien creature hell-bent on inflicting a painful death watching them from the ceiling – suddenly the Tardis is illuminated by a pillar of burnt-orange vertical strip-lights. They're new, created with a more "serious" Doctor in mind, says production designer Michael Pickwoad, who gives us a tour of the Tardis before adding that this Time Lord will be a lot of fun, too. But going by the musty books lining the walls, it does look like this new incarnation might have more of an intellectual, introspective bent. On the slightly lower level is a blackboard covered in a chalk scrawl of maths equations. To the left there's a haphazard pile of paperbacks. It all fits with the 1960s context Capaldi mentioned, but the maze of unintelligible calculations is also reminiscent of Sherlock (the two shows share a showrunner in Moffat). Perhaps with a stranger, darker Doctor this new series wants to mimic Sherlock's critical acclaim and viewing figures.

But while the show is inescapably a valuable export and commodity, it's worth remembering that Doctor Who's value also comes from the power the series holds more abstractly in the hearts and minds of its fans. The day before I visit, it's announced that Capaldi and Coleman will be embarking on a promotional world tour, taking in New York, Seoul, Sydney and Rio among others. But, in a suitably unsynchronised chain of events for a Time Lord, it will involve fans flocking who've seen no more than a couple of minutes of him as the Doctor. In some ways, that kind of devotion is impossible to live up to, but in another sense it's a massive safety net. That's how Capaldi, with a mixture of apprehension and quiet confidence, seems to be reassuring himself, anyway: "The nice thing about Doctor Who, I think, is whether people like it or don't like it you know that somewhere, somebody loves you. And will always love you. And the more everybody else hates you, the more they'll love you. 'He was my Doctor', they will say."

Doctor Who starts on 23 Aug, 7.50pm, BBC1

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