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The lead writer for Doctor Who says he would definitely cast a female Time Lord – but only if the right person came along rather than for political reasons.

Steven Moffat said he would know instantly if he had found the perfect actress to play the title role in the iconic BBC Wales sci-fi series.

He made his revelation during a talk at the Hay Festival last night when he also spoke about the reasons why Doctor Who is so successful; how Peter Capaldi is the perfect choice as the next Doctor; how he took eight months to accept the job as lead writer of the series and how the seeds for TV drama Sherlock were sown on train journeys to Cardiff.

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Scottish screenwriter Moffat, the successor of Swansea-born Russell T Davies, said he had been the one to work the dialogue to make it possible to recruit the first female Doctor.

“It will not happen when someone says we must cast the Doctor as a woman,” he said. “A person will pop into your head and when that person is a woman that’s the day it will happen. Casting is the dark arts of television – it makes the difference between a television programme and a sensation. You don’t mess around with it. It’s all about passion and aesthetics – it’s not political.”

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Moffat took over from Davies, who brought the hugely popular sci-fi hit back to TV screens after a 16-year absence, after the Welshman left for America to work on the US series of Torchwood. But although he worked with Davies on Doctor Who he said his promotion wasn’t a shoe-in.

“People assume that I was standing in line waiting for it. But I really enjoyed working with Russell and Julie (Gardner, executive producer) – I always got to be a part of Doctor Who but I also got to watch it like a viewer.

“It was a job I had always wanted but Russell was so amazing and I’d seen the workload. So it took me eight months to say ‘yes’. I was on the phone to my dad and he said, ‘Well you will say yes obviously’. He then sent me a photo of myself reading Doctor Who at the age of 10 and I thought ‘he’s right’.”

Moffat, who was also behind the TV shows Press Gang and Coupling, admitted that he was convinced that new Time Lord Peter Capaldi was the perfect replacement for Matt Smith – and not only because he was considerably older than the last Doctor.

Doctor Who locations

“Can you imagine if we had cast another handsome yet quirky young man with sexy hair? It’s (Doctor Who) a part that can be played by anyone aged 20 to 70 and you’ve got to use that flexibility not ignore it.

“Peter Capaldi just kept popping into my head and I knew he was a Doctor Who fan. He would come looming up to me at industry events and insist on talking about Doctor Who when I wanted to talk about In The Thick Of It.

“I got him round to my house and we auditioned him there as Doctor – he didn’t know he was the only person being auditioned and of course he was brilliant.”

Moffat believes that Doctor Who’s success is the fact that while it takes viewers on a sci-fi adventure, it’s also centred very much around human life.

“However epic and huge Doctor Who gets, there’s always something domestic about it. You try to look around the real world and say, ‘what can we make out of ordinary paraphernalia that could be terrifying?’” he said, pointing out that the Daleks had sink plungers sticking out of them rather than guns.

“You don’t have to go into space to be scared – you just have to go into your own bedroom at night.”

He pointed out that Doctor Who was one of the most challenging programmes to make.

“We have a new cast every week and a new bunch of sets. The amount of new stuff you have to event for every episode is exhausting but also stimulating and also brings the audience back. It’s a huge job.”

He revealed that the seeds for his other big success – the BBC Wales drama Sherlock starring Benedict Cumberbatch and Martin Freeman – were sown on train journeys to Cardiff.

He and Doctor Who lead writer Mark Gatiss realised other passengers were listening into their talks about the Time Lord so they began speaking about their passion for Arthur Conan Doyle’s creation instead.

“Over time we admitted our favourite Sherlock Holmes films were the updated ones as they seemed to capture more of the nonsense and joy of the Doyle stories.

“We said that someone is going to make a series like that and we’re going to be very cross because it’s not us.”

They eventually pitched their idea to the BBC and the rest is history.

He admitted it was Cumberbatch and Freeman, who are now both enjoying success Stateside, who have made the series what it is. “The show would have been a much smaller thing without those two playing the parts.”

Cumberbatch ,who appeared in the critically-acclaimed film 12 Years A Slave, will be at the Hay Festival on Friday and Saturday for a talk about love letters written by the famous. Tickets to the Saturday event were the second fastest-selling in the festival’s history so the other date was added.

“He’s gone from literally nowhere to huge international movie star – it’s quite annoying when you’re scheduling your show,” joked Moffat.

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