Shetland star Ashley Jensen has revealed she doesn't like to know who the series killer is as she prefers to do her own detective work.

Her character DI Ruth Calder is back on our screens next Wednesday in the 10th series of the BBC One drama.

While filming on the island she watched cold case crimes on TV and believes this helped her to detect the killer in this year's series.

"I like being just like the audience, enjoying how much it all unravels," she says.

"I was better at working out who the killer was this time around - clearly all my cold crime docudrama research has paid off."

DI Calder is joined in the wilds of Shetland by new and old cast members, including Alison O'Donnell as DI Alison "Tosh" McIntosh.

The two detectives have now settled into a solid working dynamic and a blossoming friendship.

Jensen and O'Donnell believe their warm on-screen relationship is helped by the fact they are both very down-to-earth and love to have a laugh.

"I think Calder and Tosh are really complementary," O'Donnell says. "They have quite distinctive policing styles but between them they've got everything covered.

"The comedy flourishes that the writers give us are a lovely counterpoint to the darkness of the drama and we always have a lot of fun with those."

The pair are in no doubt that the breathtaking landscape of the island is a definite pull for viewers and immediately recognisable as being Scottish.

"The landscape of the isles is pretty much purpose-built for drama and the breathtaking magic and beauty of the Isles is our USP [unique selling point]," O'Donnell says.

"But I also believe it's the fictional world that we've built around that.

"Our wonderful ensemble of returning characters, the epic stories, all the characters we meet along the way, the darkness and intrigue with a pinch of humour and a big dollop of Scottish trustworthiness."

The actors also believe the popularity of the series is because people love to be taken on a journey.

This time, the pair find themselves investigating a murder of an elderly woman in the isolated hamlet of Lunniswick.

As they begin to unravel life in the close-knit community, shocking, long-buried secrets rise to the surface with unexpected consequences for everyone - including the police team.

This is the first time Shetland has taken over a real-life village and transformed it into a fictional one.

Lunniswick is actually a place called Levenwick which is on the road between Lerwick and Sumburgh airport.

O'Donnell says: "Lunniswick is typical of Shetland, scattered houses lining a beach, the neighbours well acquainted with one another and the land they live on.

"It is rich in history, relationships and secrets which I really loved as a central focus of the mystery. There was so much to play with, so much to unpack."

Jensen adds: "This series we've kind of become a little more insular rather than the expansive locations of last year.

"We've really come into the nucleus of the small hamlet of Lunniswick and used a real-life town as our location. It's very much a small community where everybody knows each other's business."

She adds: "The murder itself is a peculiar death as the victim is an older woman and she's positioned in a rather macabre way.

"What makes it worse is she's outside her house, exposed to the elements and she's been out there for a few days. So, obviously, because it's a small community, everyone's a bit suspicious.

"For me, this series is the best storyline out of the three I've been involved in. I love the detail of this one and the fact that all the locals are suspects in the murder case."

Jensen says a fascinating element of the plot is that the personal lives of the police officers become embroiled in the case.

"This has been interesting for us all to play," she says. "There are relationships in the station which become slightly fraught and fractious because of the specifics of this case."

She adds: "Cleverly, what the makers have done is keep the essence of what the show is. People love the environment, they love that expansive landscape - it's so alien but also recognisable."

Jensen says she enjoys filming on location because "it feels more alive".

"You're actually in a house, you're beside the water and you can smell the sea air outside and I find it helpful to feel the damp, feel the cold even though it can be difficult," she says.

"The fact your hair is going all over the place can add to the wildness of the show and the elements are bigger than you and as much a part of the show as the actors and the storyline."

Shetland star: 'I like to work out killer as plot unravels'