One of the last things you’d associate with Saga Norén, the curious and compelling Swedish detective now back on our screens in the third series of The Bridge, is a love of fine food: utterly work-focused and oblivious to life’s creature comforts, she’s usually glimpsed scooping something unidentifiable and microwaved from a tray, refuelling before she interrogates the next suspect. But it’s not just about efficiency. Saga’s pronounced difficulty in navigating personal relationships, combined with a traumatic family background that we are gradually learning more about, mean that eating as a communal activity is entirely alien. When, in an earlier series, she found herself accidentally having dinner in the bosom of her police partner Martin Rohde’s noisy and vivid family, she seemed utterly bewildered – not that it stopped her pointing out that she didn’t like the food.

Sofia Helin, who plays Saga, clearly tends more in one direction than the other: in the gaps between projects, and when she can get a break during filming, she likes nothing better than to settle into domestic life: “A favourite situation at home,” she tells me over lunch in Soho’s Dean Street Townhouse, “would be me in the kitchen listening to a good podcast or radio show, and the children playing something, and all of us together at home. The ideal.” Helin’s husband, Daniel Götschenhjelm, is a former actor who is now a Church of Sweden priest, and the couple have two children, a son, Ossian, who is 12, and a four-year-old daughter, Nike. After eight years in the same place, they’ve just moved into a new apartment in Stockholm; all they need now, she says, is a gap in the schedule to unpack all the boxes, put up the pictures and install the television.

It’s not surprising she hasn’t had much time for decorating; the last few years must have been a whirlwind. An acclaimed stage and screen actor in Sweden, Helin was just beginning to get back into work mode after Nike’s birth when The Bridge, a crime drama which takes its name from the Øresund Bridge that connects Sweden and Denmark, came along. “She was just 11 months,” Helin recalls, “so it was a big difference.” Her solution was to take her daughter along with her, which, she says, worked out, despite obvious sacrifices on the sleep front: “You know, when they are one year, they wake up at night, so I had her waking up in the night, and then shooting during the days, and then I went home to my son to try to catch up with him. I was kind of exhausted after that.”

Things have clearly settled into more of a rhythm now, and she champions the virtues of Sweden’s inclusive and forward-thinking childcare system, describing an impromptu visit she once made to her daughter’s kindergarten in which she discovered that “the teacher was reading haiku and the children were painting. I was thinking, this couldn’t be better!”

Added to the normal pressures of juggling a working life that frequently takes her away from home for lengthy periods – currently she’s working on a new project that’s shooting in Prague, and when she’s filming The Bridge she spends four days a week in Malmö, where it is partially set, and three days back at home in Stockholm, nearly 400 miles away – are the specific challenges of playing a character like Saga. The detective is frequently described as having some kind of autistic spectrum disorder, such as Asperger syndrome, although it is never explicitly pinned down on screen. In the first episode of the new series, forced to work with an unfamiliar detective, Saga raps out a list of rote personal questions; when asked what she’s doing, she replies simply, “small talk”. Sure, acting’s a job, but it must be tough to disengage from playing that singular a role?

“It is,” Helin agrees. “But I have techniques for that. And I also see life coaches and therapists or try to do physiotherapy, because I see that almost as a part of my work, to work with myself. Because if I am someone else for so many months, it affects me, physically, in my brain, in my being. So then I have to take care of that, and make myself more like me again.” Sometimes that even involves going on a retreat, all part of the process of preparing for the next thing, “to try to stay open, not to close yourself”.

She says that the international success of The Bridge took her by surprise, not least because of the extent to which it raised her profile. Her own country is not big on celebrity culture: “In Sweden, you’re not supposed to think you are someone.” And this new series brought with it an additional factor: in the previous two, much had rested on the dynamic between Helin and Kim Bodnia, who played Danish detective Martin Rohde. At the end of the second series, Rohde was himself taken away in a police car, having avenged the murder of his son; and in between the two series, Bodnia decided that he would leave the programme. Rohde – burly, twinkly-eyed, fallible and somehow able to forge a bond with Norén – was much beloved by the show’s fans. How hard was it to adjust to her co-star’s absence?

“The process was painful,” says Helin, but she adds, “For me, it was a challenge. I’m sad and I will miss him, but I can also see that the show got new energy from this.”

I ask whether Saga, such a strong creation, is still developing as a character? “She is, constantly. But she can’t change. You can’t make her have social skills, because she would never know how to do that other than in a very mechanical way. But we are putting her into very different situations, so she thinks she’s in a place where she’s safe from her past, but she’s not. It’s coming back to her.” In this series, her mother, whom she has not seen for many years, and who Saga thinks has Munchausen’s syndrome by proxy, appears out of the blue, causing Saga to bolt, terrified, into her apartment and begin compulsively straightening the spines of her books for relief.

Saga is also recognisable for two other things: her appearance – wild hair, face free of make-up and leather trousers – and the classic green Porsche in which she roars from crime scene to police station. Helin has said many times how little she likes the Porsche – “It’s just not easy to drive, and it’s cold and uncomfortable” – but Saga’s look presents another issue, which comes up when, in an unlikely twist, we are discussing kale. Knowing of its health-giving properties, she started off by boiling it, and then tried cooking it in the oven, “because I read somewhere that you can make crisps out of it with olive oil and salt”. Any good? “It tastes like shit!” She laughs, and then corrects herself. “Not like shit – it tasted bad, really, really boring.”

Not like crisps, then? An emphatic no. “I love crisps.” Do they fuel her on set? Yes, she says, but “when you keep on shooting for nine months, you can’t have crisps every day. Because then you can’t fit in the leather trousers!”

When she’s at home, she prefers to stay there rather than eating out, and cooks simply and healthily, roasting root vegetables and grilling chicken. In extended periods of downtime, she might make bread. And her lunch habits suggest that she is a person of naturally abstemious appetites – a salad, a little bread, no pudding – until the very end, when she catches sight of a plate of beautifully fashioned chocolate truffles making its way to another table. She has, it appears, something of a sweet tooth. We order some up, only for her to be called away to another room, another interview. I insist she takes them with her. It seems only right that she should have something nice to sustain the continuing saga of Saga.

The Bridge, BBC4, Saturdays at 9pm and 10pm. The Bridge Season 3 and Trilogy Boxset are available on DVD and Blu-ray on 21 December