Luther star Steven Robertson on playing a good guy at last in Shetland

“I’ve met so many grisly ends,” he says, laughing about the likes of BBC drama Luther, in which he played both of the Millberry twins, a murderous duo who decided their victim's fate on the roll of a dice. “It’s just so nice that in Shetland I’m not being bludgeoned or macheted to death.”

Shetland is back for a third series this week and its fan base is growing. Was returning to Shetland (the series) like putting on a warm overcoat – or did Robertson feel a heavy weight of expectation, given he grew up on the islands?

“A little of both,” says the 39-year-old. “When we started with series one I did feel a bit of personal responsibility but I guess I was caught up in the excitement of it all. By series two however I really felt the responsibility. I so wanted the series to work, for the islands. It’s been great to see people I knew growing up become involved in the show and work on and use TV industry words. It’s funny, but in a really nice way.”

Robertson didn’t grow up hoping to find acting fame. In fact, he’s a mainland away from being the stereotypical stage-struck kid. “I grew up learning traditional stories and loved them,” he says. "I was dyslexic and to hear stories told was amazing, and then I began to tell them in my own way. But I never thought about becoming an actor. I was an economic migrant.”

On leaving school, Robertson became a dairyman, milking cows for two years until he lost his job when the farm moved from dairy to beef. His next job was as an apprentice violin maker. “You have to have a large skill set, living in Shetland,” he says, laughing. “But this got me involved in folk festivals, where I was asked to introduce acts and do some compering. At this point I became a bit obsessed with stand-up and tried some. I was appalling, as it happens, because I tried to copy Billy Connolly and Eddie Izzard.”

By now aged 21 and bitten by the performance bug, Robertson studied performance at college in Kirkcaldy with the idea of becoming a full-time compere. But then something unexpected happened: his tutors recognised a rare talent and persuaded him to study straight acting. “I’d never even thought of it. It all felt very strange. But it worked. And I went on to apply to the Guildhall drama school in London, which was like starting again, but it was fantastic.”

Robertson’s tutors had spotted well. On leaving Guildhall he was snapped up by the Royal Shakespeare Company. From there he landed key roles in the likes of Chekhov's the Seagull which one critic described as an "arresting" performance.

Television came banging on his door too, and his first major film role arrived in 2004 when he starred alongside James McAvoy in Inside I’m Dancing. Did his Shetland accent, heavier than a wet Fair Isle jumper, have to be binned?

“Yes," he admits. "At drama college you are encouraged to keep your own accent, but you can’t use it in anything else. I had to work to remove it because if you don’t, you won’t work. It’s only now with Shetland I’ve been able to use my real accent ... well, it’s almost real.”

Robertson has enjoyed the dark parts he’s been cast in, yet he claims playing an ordinary person is harder. “If you play the twins in Luther your behaviour is so extraordinary it will stand out. But Sandy simply wants to be good at his job, to improve as a detective and have a nice relationship. And if Sandy makes a decent cup of coffee convincingly on the screen it won’t have the viewers on edge.”

Yet you need such characters in a series such as Shetland. After all, not everyone can run around harum-scarum, having wild affairs or getting beaten up. “That’s right," says Robertson. "Life is full of Sandys. We need them.”

The industry also needs actors who are natural chameleons, who can play slithery oddballs in the likes of Doctor Who. Robertson throws in an aside about his role in the science-fiction series as Richard Pritchard, which though not very actory is frighteningly honest. “It took me a while to get the tone of the piece, what my character was all about, and I got one or two takes more than everyone else in the room.”

Is that because he couldn’t draw upon real life to get his character right? “That hits the nail on the head,” he says. “I like to have a personal connection with my characters. You like to have personal experience to call upon.”

From listening to Robertson talk about his wife Charlotte and their baby Jemima you guess he could easily play a devoted father. “I could,” he says, grinning. “I’ve got the experience. But what I love about Shetland [the series] is the challenge of playing an ordinary guy, and now seeing so many islanders appear in it and work on it.

“They’ve come to realise how filming is very demanding and tricky. They’ve come to realise that acting is more than, to quote the line from Blackadder, ‘getting drunk, putting on a silly hat, and trusting your luck.’”

ALISON O’Donnell clearly thinks a great deal about her role in Shetland as policewoman Tosh, who could drink for Scotland and throws up over dead bodies. In fact she deliberates to an unimaginable degree. “It is very intimidating when you turn up to film and here’s why,” she says. “There’s a feeling the camera is like a time-travel device and that you are standing there, doing something in the moment, but people won’t see this for another eight or nine months.

“It’s almost as if the two moments are happening simultaneously. Obviously, that’s not a helpful way to think when you’re standing there filming, but I can’t help but be aware of that.

“In one version of time you can see all the nuts and bolts of television production around you, you can feel the cold of the film set, but in the other version, the glossy version eight months ahead, you are being watched in someone’s living room. It can be a bit surreal.”

Such a sense of awareness – and responsibility for the job at hand. And it’s not as if O’Donnell is lost somewhere up her own space-time continuum. Far from it. She laughs easily and loudly in conversation but given the focus she has on her work it’s no surprise that Alison MacIntosh has become such a pivotal character. The Motherwell-born actor, however, is modest about her role. “I am very, very lucky to be involved in a series which has been reprised twice. When Shetland started out it was made in hour-long stories, and those four stories were all adaptations of Ann Cleeve’s books, a more traditional format. That worked. But it does have its limitations. You have to establish new characters every couple of hours, and the set-up time means there is less space for other things.

“Now, with a running storyline, once you’ve met the key players you can develop the characters. I get to do more with my character, but it’s also more of an ensemble piece.”

Do actors truly want ensembles, I ask. Surely they want the spotlight to shine on them and them alone?

“That’s a very cynical thought,” says O'Donnell. “No, in all honesty, we all get to develop but it allows for my character to reveal more about herself.”

What will we learn about Tosh? “I’d like to think she has become more confident. When we met her in the pilot she was making silly mistakes, turning up on the job half drunk and vomiting over the murder victims.” She did, but it humanised her. You never saw Alex Norton in Taggart be sick over a corpse. “More’s the pity,” says O’Donnell, laughing.

It was good, I suggest, that Tosh looked as though she’d been dragged through a hedge backwards by a team of plough horses. “I thought so,” she says, “but I think to have kept her in that place would have undermined the job. And you have to honour the relationship with Perez [played by Douglas Henshall]. He has fast-tracked her to success so she has to become a bit more responsible and it’s more about solving the murder.”

Ah, the case. The storyline. What can O’Donnell tell us about that? “Well, we become more personally involved in the journey and Tosh goes on a personal journey as the story progresses.” What happens to her? Does she fall in love? Does she fall by the wayside? Does she fall into a vat of lager? “I can’t really say,” she says.

An easier question, then. Did she always want to act? “My school, Dalziel High, had a great drama department and I really loved this world. But I was also being told I should go to university.”

O’Donnell did exactly that and studied European law. For all of three months. Was there an epiphany? “There was, actually. I don’t mention it because it sounds as though I’ve made it up. But I was standing outside the university library when I heard a group of guys talk about their dads being lawyers and how law was in the family. Then one of them turned to me and said: ‘What about you, Alison? Have you always wanted to become a lawyer?’ And without thinking I said, ‘No, I’ve always wanted to be an actor.’ In that moment it all came to me. I began to think, ‘What am I doing here?’

“At that point, I got a really bad cold and missed lectures for a few days. But in fact I missed them for ever because I didn’t go back.”

Her parents’ reaction? “They’ve both been great.”

It’s rather a dramatic life change from European law to showing off. “I think you have a distorted view of the acting business,” she says, grinning. Not at all, I reply. You have to have respect for the very best in the business but isn’t it still showing off?

“I don’t know,” says O'Donnell, her tone more serious. “I like to think it’s about making a connection. And that feels really special.”

O’Donnell is 33 years old but looks much younger and clearly has a talent for her profession. Her theatre work, mostly in new writing, has attracted critical acclaim but she had to adjust for television. “I was really green when I first showed up and thankfully Dougie proved to be a fantastic mentor. I was made to feel that no question was too stupid to ask.”

What is she doing now to give a better performance? “You realise less is more. In a lot of theatre your performance is about making sure you can be seen in the back row.

“But you learn that television picks up every tremor, every twitch, especially in HD. And here’s the thing. Sometimes you just have to think the thoughts and the camera will pick it up.”

Will the camera pick up the fact she’s thinking eight months ahead?

“I hope not,” she says, laughing. “I want to look like I’m in the moment.”

The third series of Shetland begins on BBC One on Friday at 9pm.