Inside a large wooden cocoon, in the heart of Roath Lock Studios in Cardiff, is the Tardis set.

It is a special place and, remarkably, a place to which Peter Capaldi comes for quiet moments to find a bit of peace on the manic set.

Visiting Cardiff during the filming, it is a rare pleasure to be allowed a few moments to enjoy these iconic surroundings. After asking a few perfunctory questions to the show's executive producer Brian Minchin about the new colour palette (it's a more metallic orange), the new, under-the-counter lever (what fun!), and the chalkboard on which Capaldi's Doctor will be scrawling his workings out in longhand - he loves chalk, as demonstrated early in his feature-length first episode - it's play time.

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Buttons are pushed and levers pulled, as tradition dictates. Lights flash on and off, dials whirr. We are really flying this thing. Then it's time to get serious. The Doctor is approaching…

In a non-descript office, it's time to come face-to-face with Peter Capaldi, for the first time since he was unveiled as Matt Smith's successor.

The new era of Doctor Who is upon us. Capaldi and Jenna Coleman will take the Tardis on a world tour when filming is finished to destinations including South Korea and Australia, something unthinkable in the past. So how is he feeling about finally unleashing his Doctor on the world?

"It's really exciting, we are reaching the end of filming now, and it's all going mad," he says. "It has been the most intense job I've ever done. This is our 30th consecutive week, and the Doctor, thankfully, is there every day. I have loved it; this has been a wonderful experience. But it is a marathon."

He describes the world tour, his unveiling live on BBC1 in a special show presented by Zoe Ball, the glitzy London premiere of the first episode - and the glowing reviews - and the gradual drip feed of trailers, pictures and teasers about the new series as "all part of the showbiz. It's all fun, isn't it? It makes it even more of an event."

If the set of Doctor Who can reduce even the most cynical hack to an excitable, child-like state, then Capaldi, a lifelong fan, was not immune.

"I even went into work on one of the rare days when I wasn't needed on set," he explains, "because I saw 'Dalek attack' on the schedule. I've never seen so many grown up, professional people transported back to their childhood."

Capaldi recalls the early Doctor Who adventures, starring William Hartnell, vividly. He was such a fan of the show that he contributed articles to fanzines as a teenager, and had letters published in Radio Times. The 56-year-old actor has not, however, been saving up his ideas of how to play the Doctor for half a century, despite showrunner Steven Moffat teasing that Capaldi has "been preparing for this role since he was six years old".

"When you are a kid you are just whoever's Doctor is on. Whether it be William Hartnell or Patrick Troughton, Tom Baker or Jon Pertwee," says Capaldi. "You don't think, 'my Doctor would do this', because you are not an actor, you are just a kid! So I never had any concept of a Doctor that I would play - and even when I became an actor, it was never something that I thought was ever likely to occur to me."

His first day of filming was a career highlight, he says.

"I had to step out of the Tardis into a whole new world. It was so exhilarating," he says. "You can't worry too much about setting out who your Doctor is, otherwise you wouldn't get on with the business of acting that particular story.

"It is quite scary, but fantastic to find yourself in that position. I was literally stepping into Matt Smith's shoes. And his trousers! It was frightening, because everyone adores Matt. But the first time, opening the Tardis door and stepping out - that is a special day."

So what can I say about Capaldi's Doctor, on the evidence of Deep Breath, the series opener, which screens next weekend? Well, Capaldi brings a great intensity to the role. He is less bouncy than David Tennant, there's a real anger in his eyes at times. He is funny, but also more short-tempered than any of the previous modern Doctors. He also speaks with Capaldi's Glasgow accent - something that, alongside his age, and the fact that he has previously appeared as a different character in Doctor Who, is acknowledged in the episode.

"The Doctor has had an English accent for years so the idea that he hasn't had an accent is ridiculous," says the actor. "I wanted to bring the role closer to me. Putting on an accent would be like putting up a barrier between me and the Doctor."

Capaldi's summation of his Doctor is vague bordering on hopeless. "He is different from the others. And yet he is the same. That is utterly useless, isn't it?" he laughs.

He's right, though. In the opening episode, beyond battling a few "rubbish robots from the dawn of time", we also witness the 12th Doctor finding it hard to come to terms with his new identity - just as his companion, Clara Oswald (Jenna Coleman), finds it hard to let go of her younger-looking Time Lord.

"The reason I am vague is not because I'm being coy, but this Doctor struggles to find himself. He struggles through all this wreckage to find himself. My Doctor is still quite a mystery, even to me."

There is no mystery to the show's continued success, though. Not for its new star anyway.

"The show is brilliant, but it has had its highs and its lows and will continue to have its highs and its lows.

"That is part of the wonderful spirit of it that I love," he says. "I always think it is almost like a Grimm's fairytale with a sci-fi body. He appears and takes people deep into the forest, you know, where there are monsters. But he will deliver them back to safety at the end. I think that is an attractive thing. And everybody loves monsters. Any show with monsters in it works!"

Capaldi prepares for a day playing the Doctor by listening to Murray Gold's "fantastic" music from the show. "His new music for my Doctor is stirring - so I listen to that. He invited me down when they were recording it just round the corner from here, and it was fabulous to be in the room as the orchestra played the music," he says.

Having taken episode one on a whistlestop tour of the globe, Capaldi is finally unleashing his vision of the Doctor on to the public.

"I have only just realised how appropriate the title of the first episode is. Deep Breath. I am going to take a deep breath and see what happens. I hope people like it.

"The great thing about this show is that I know some people will love me forever and I will be their Doctor. And if everyone else hates me? Well, then the people that love me will love me even more."

I wish him luck. "Thanks," he grins. "And give my regards to Glasgow…"

Doctor Who, Saturday August 23, BBC1, 7:50pm