Sir Kenneth Branagh and Henning Mankell are making small talk. And it’s a long way from the dark, cold, Swedish world of Kurt Wallander, the fictional character who brought them together. “If by any chance you need a little house, we have a house in the south of France in the old Antibes,” says Mankell, as they both settle into armchairs. “You can always make a call.”

“Well, I will make a call, that’d be lovely,” says Branagh, either delighted or very polite. “We went there for the first time, to St Tropez, for a week last year, and we were amazed that it had such character still.”

“In the old city, where there are no tourists, there are people who go to work, go to school,” says Mankell. “But if you walk 150 metres, you are round with the tourists.”

And then, barely missing a beat, they settle down to business. Mankell is in London for the launch of the third series of BBC1’s version of Wallander, in which Branagh stars as the eponymous detective. Mankell wrote the massively successful novels on which the series is based, and he retains a hand in the BBC production – “Henning is willing to talk when we wish to ask questions,” explains Branagh. Hence Mankell’s easy familiarity with Branagh, with whom he has now been working (on and off) for four years.

For Branagh, who made his name in films and on the stage, sticking with a series for this long is a real departure. So how did it feel, to return for another three feature-length episodes? “Well, I’d love to know what Henning thinks,” says Branagh diffidently. “But I was saying to him earlier today – and I really meant it – that I was excited to come back. Because it’s unusual, in my creative life, to come back to a character like this, who is on television – but with these unusual ambitions of trying to match up to what Henning suggested so wonderfully in the books.”

Indeed, despite having been knighted in the Queen’s Birthday Honours list just days before this interview took place, Branagh remains resolutely deferential to Mankell as they talk.

Physically, they are an unmatched pair. Mankell is taller and, at 64, older. He has the air of the artiste, and wears a black suit over a jazzy black-and-white print shirt. (A triangle of his tummy peeps endearingly out from underneath.) A pro-Palestinian activist who spends much of his time running a theatre company in Mozambique, Mankell also has striking Heseltinian hair, though whiter and more unruly.

Branagh is 51, and his hair is also now largely grey. But it is neatly cut, and he wears a sensible blue linen suit with a dark shirt. Branagh has none of the booming, overly precise verbal tics that some actors develop, and that perhaps betrays his mastery of his craft.

Even Branagh’s knighthood (“a surprising and wonderful thing”) is, he says, a testament to the work of others. “You know, the first thing that flashed into my mind when I received the letter, was this image of the credit sequences of the films I’ve made, which are full of hundreds, sometimes thousands of names,” he says, of a career that includes playing Henry V in 1989 and directing last year’s Marvel superhero flick Thor. “I just thought that this ends up being an acknowledgement of all of that.”

But despite Branagh’s modesty (“Just Ken, please,” he admonishes good-naturedly when addressed as “sir”), Mankell is quick to return his compliments. “I have never talked about this before, but I have been thinking about the fact that Kenneth is a very well-trained classical actor,” he says. “Last year, I saw Ken perform in a Chekhov play here in London, and I think he carries this with him into movie-making – this capacity of showing, always, how complicated human beings are. Because if you don’t have that capacity, you can’t do classical theatre. And it makes a difference, because there are other actors who only do film.”

The character of Kurt Wallander may not quite be Hamlet, but he is certainly a complicated human being. In his blank Swedish way, he combines first-rate detective skills with an almost complete inability to maintain personal relationships. More deliciously for the viewer, those personal travails seem to stem from the emotional damage inflicted by the horrors of his police work.

“In one of the new episodes, Wallander says to his colleague, ‘I think I’m really quite a cheerful person,’ which causes great hilarity when people see it,” says Branagh. “But I agree with him! I think he is quite a cheerful person. It’s just he doesn’t often get the chance. But what he does engage with is that he takes his life quite seriously. He's not embarrassed by thinking – and occasionally expressing himself – about what you might call “deep things”.

Filming in the southern Swedish country of Scania, says Branagh, adds to Wallander’s solitude. “To look out of a car in Scania, you see a painting on the horizontal – one windmill, one tiny farmhouse, acres of beet or grass,” he says. “Everything is dramatic, framed, composed... God somehow in a little cinema laboratory decided it’s very good if you keep things flat.

Wallander’s unflinching introspection –garnished, of course, with large helpings of violence and gore – proved a literary hit for Mankell. From the publication of the first book in 1991, it multiplied into a series of 11 novels (including one with Wallander’s daughter, Linda, taking the lead). The books have been translated into more than 40 languages, and sold over 40 million copies worldwide. Today, those basic ingredients have mushroomed beyond Mankell’s pen into a hugely successful genre, known as many to be one of the literary masterworks of the Nordic Noir. It spans Stieg Larsson’s Millennium trilogy (and their film adaptations, including The Girl with the Dragon Tattoo), as well as the hit Danish TV detective series The Killing and the more recent Swedish/Danish co-production, The Bridge.

“In the 70s, in Sweden, we had suddenly a very good tennis player called Bjorn Borg,” says Mankell. "He came out of nowhere. And then all of a sudden we had many good tennis players. Nothing succeeds like success. Obviously I have been an inspiration for many writers. I think in that sense you could say that I have been a sort of locomotive, Wallander is the locomotive. The Killing, I watched, it is very good. (Danish political thriller series) Borgen is also very good. But then, I can assure you, there is a lot of s**t.”

Mankell is too diplomatic to name and shame, and Branagh has only praise for The Killing. “Yes, I enjoyed it hugely, I saw both series,” he says. “The thing that I was fascinated to watch is that it allows quite a lot of room to simply observe the characters thinking and being. I was happy not to be told everything, even though it’s also very suspenseful.” Branagh also acknowledges that the long running time of The Killing – the first series lasted 20 hours – helped create that atmosphere.

But, he says, British TV drama should only adopt a similarly lengthy approach if the right series comes along. “Only if you find that subject matter, only if you find the people who want to write it,” he says.

“And we have different writers – Jimmy McGovern’s an example, with something like The Street, exploring things in an important way, not maybe at that length.”

Branagh points out, though, that each episode of Wallander lasts for a full 90 minutes. “People are ready to listen to the silences, as long as we are earning them, and it doesn’t become a mannerism,” he says. “For me, every Wallander is a one-off experience. It could be more raw, or more real, or more simple or more of a chance to let the audience do the thinking and feeling. We take a long time in the run-up to each series. I read the final Wallander novel The Troubled Man not long after it was published. Then my wife read it a little while ago, and loved it...”

Mankell  interrupts, surprised. “You’re married?” “Yes,” says Branagh, then quite hotly, “Yes, I am!” “Yeah?” “Yeah, yeah. Yeah.” “You were not married last time we met,” says Mankell, firmly. “No, I was, I was,” counters Branagh, who wed art director Lindsay Brunnock in 2003. “Congratulations!” says Mankell. “Thank you very much,” says Branagh, before ploughing on.

“This last time was the first time she came out to Sweden – thank God – and created a great house for us,” he says, and then, finally, returning to his theme: “We had ended up talking about The Troubled Man, so I’d already had two hits of experiencing that.”

As well as that laconic pace, says Mankell, the best Nordic Noir has a conscience. “I know that these guys who write The Killing, they do the same as I do. They ask themselves, before they start writing, ‘What do we want to tell about our society?’” he explains. “We are living in a world where some of the most dominant businesses are selling weapons, drugs, and trafficking. Out of the five biggest businesses in the world, three are absolutely based on criminality. To hold out a mirror of crime, to talk about society today, it is a very efficient way to tell a story.”

And for the British version of Wallander, says Branagh, the same resonances hold true. “We think, ‘What is a version of the current Swedish attitude to, say, immigration or prostitution or the severity of criminal punishment?’ Inevitably, all of those reflect back into our own system, and it makes us compare and contrast,” he says. “There’s always something to think about, in terms of problems that are dark and important and immediate and scary.”

Series 3 of Wallander begins tonight at 9:00pm on BBC1