Timothy Spall's new crime drama filmed entirely in Wales

When the former star of a police crime show moves into town, the local real-life detective can hardly believe her luck.

So begins the plot of the BBC's new comedy-crime drama Death Valley, filmed entirely in Wales, in which Timothy Spall and Gwyneth Keyworth pair up to portray an unlikely crime solving duo.

Wales has become a popular destination for film and TV production in recent years, but often stands in for other places.

The action in Death Valley, however, is firmly located in Wales - albeit in a fictional town - and embraces the country's culture and landscape.

"It's lovely to hear Welsh being spoken... It's kind of a celebration of Wales as well," said Spall.

Spall portrays John Chapel, the favourite TV character of Det Sgt Janie Mallowen, played by Keyworth.

Det Sgt Mallowen idolises Chapel after his TV show Caesar helped her through one of the darkest times in her life.

But she begins to discover that Chapel the man is not quite the same as Chapel the actor.

The show's writer, Paul Doolan, admitted to being a "bit of a murder-mystery obsessive".

"I liked the idea of a TV actor who played a detective, and a fan of them, and the awkward relationship, but I couldn't really get beyond what else was funny about that.

"Then I thought 'what if they solve murders?', which is all really meta, and then started plotting it based around that."

In the show, Spall's character first comes across Det Sgt Mallowen when his neighbour is murdered and she arrives to investigate.

"All relationships, you need that conflict, you need there to be stakes and you need a power dynamic," said Doolan.

"Having him being famous and her being a fan, you only need to know that and straight away you get what all those dynamics are.

"The physical differences, the generational differences, he's English, she's Welsh – there's just piling as many opposites in together and have them be spiky to each other."

Apart from one 1980s role in a BBC Play for Today as a "rural police sergeant who had to do all the work", Spall, star of Mr Turner, Harry Potter and the iconic Barry in Auf Wiedersehen Pet, has never been in a "whodunnit", and admits to not having watched much in that vein.

"But when they're great, people love them," Spall said on set in Cardiff.

"What I love about this is we're doing all that. There are moments in it where you're aware of it being a whodunnit, but it subverts itself by the eccentricity of the duo and the circumstances they find themselves in.

"Paul's been meticulous in how everything's planned out. So really, if you don't like comedy, or you don't get it, you've got the [crime], and if you like comedy and don't get the crime, you've got that as well."

Being an actor playing an actor gave Spall "some opportunities to illustrate that pretentious side of it and if I'm taking the mick out of anybody it's myself", he laughed.

He was full of praise for his co-star Keyworth, of Misfits and Hidden Fame, whom he first encountered when she starred in a West End production of To Kill a Mockingbird alongside Spall's son, Rafe.

"I thought, bloody hell, she's good. Then a couple of years later here we are in this odd-ball double act," he said.

Her character, Det Sgt Mallowen, is "very practical", he said, adding: "She's procedural, she's a policewoman and has to go through all sorts of things.

"They are quite candid to the point of rudeness and offensive with each other but they always somehow are reliant on each other. They'll never admit that they have affection for each other."

Behind the humour, there are darker sides to the show.

Det Sgt Mallowen lost her best friend to suicide aged 18 and the drama portrays how she deals with that trauma.

Keyworth, from Aberystwyth, thinks plenty of people can relate to using escapist TV to cope with tough times - as her character did with Caesar.

"It's like her hero has come home to roost, but then she has to deal with the reality of [Chapel] is not actually Caesar, and in fact can be a little bit annoying.

"They've both been really lonely for a long time and there's a level of kindred spirits that meet.

"Then ensues a lot of comedy because they have very different approaches to life."

The supporting cast include Gavin and Stacey's Steffan Rhodri, Sian Gibson from Peter Kay's Car Share, Alexandria Riley of Silo and The Pembrokeshire Murders, as well as additional writing from comedian and actor Sian Harries.

"Every year we've got these brilliant Welsh actors turning up," said Spall.

"It's lovely to hear Welsh being spoken. It's lovely to have the whole lilt, that wonderful sound. It's kind of a celebration of Wales as well."

Both Spall and Keyworth were fulsome about the wonders of the Welsh countryside, with Spall noting the convenience and speed of transferring from the BBC studios and urban locations such as Insole Court in Cardiff to more rural ones, such as the Bannau Brycheiniog National Park, for filming.

"We see lots of pretty Wales [in the show]," said Keyworth.

"There's an episode with a walking group we get to see some really, really picturesque locations that just make you go, Wales is so beautiful. There's a waterfall that's particularly stunning."

The seaside town of Penarth in the Vale of Glamorgan also gets a shout out.

Keyworth adds with a laugh: "I love a pier, coming from Aberystwyth myself."