Sir Kenneth Branagh was doing “Scandi” before it was invented.

His characterisation, in English, of melancholic Swedish detective Kurt Wallander ignited a drama phenomenon that has since seen viewers obsessing over shows such as 'The Bridge', 'The Killing' and, more recently, 'Trapped'.

All were subtitled at a time audiences would have naturally turned away from reading dialogue rather than listening to it. Now subtitles are less a distraction and more a prerequisite.

The Northern Ireland-born actor, producer and director is one of the busiest people in show business and it is perhaps understandable the 55 year old initially looks a little tired, as he’s in the middle of a West End run in comedy 'The Painkiller'.

However, Kenneth, one of the country’s most compelling actors, is soon in full flow as he chats about returning for a fourth series of 'Wallander' and the character he loves playing. And he is just as you would imagine – polite, funny, modest and armed with a battery of entertaining anecdotes.

We discuss the powerful, concluding episode to the three-part series, which opens in Africa where Wallander’s Swedish author, Henning Mankell, spent much of his life.

Kenneth tells me, “I have to say that in this Wallander he’s happy. We’ve seen him not deal very well with the divorce or deal with violence in a job that he seems to take very, very personally, in ways, frankly, that Swedish police, as they’ve said to me, should not. In the training, apparently, one of the things you have to learn is to switch off in the middle of a manhunt or if a child goes missing. You have to check back into your life.

“He also has a difficult relationship with his daughter, but is completely charmed by his granddaughter. He seems to be relishing that chance of being able to be himself a little more. Unfortunately, it comes at the same time as he has this genetic connection with his father, which we saw in the first series.”

Wallander’s father suffered from Alzheimer’s disease and fans of the series will remember how brilliantly the actor captured that harrowing episode.

Now dementia casts its shadow again, with Kenneth turning in an extraordinary performance. There are echoes of King Lear when Wallander is found on a hillside near his house by his daughter. Not knowing where he is, he is in a state of acute distress.

“Kurt is so resilient and determined that when you see anyone in this state with all of that removed, it’s very affecting,” says Kenneth. “In 'King Lear' he asks Goneril, ‘Are you my daughter?’ So Shakespeare understood that dislocation.”

The actor’s illustrious career has seen him play some of the Bard’s greatest roles and achieve awards and accolades across film, television and theatre. 'Wallander' won the best drama series Bafta in 2009 and best actor award for Kenneth the following year. He has also been nominated for five Oscars – not bad for the son of a plumber and joiner. “I feel Irish,” he says.

“I don’t think you can take Belfast out of the boy.”

Kenneth emerged from London’s RADA (Royal Academy Of Dramatic Art) alongside a promising generation of actors that included Alan Rickman and Juliet Stevenson. He was married to award-winning actor Emma Thompson for six years, then had a relationship with Helena Bonham Carter. He married art director Lindsay Brunnock in 2003.

Kenneth believes it is important that 'Wallander' has always been shot on location in Sweden.

“Mankell uses the sky and the weather to reflect and resonate with the story’s mood,” he explains. “When I first went there we were going to shoot in another town, but the landscape made you imagine things inside it. When you drive over the Øresund Bridge, the land goes flat, it stretches away from you and it feels like everyone has cleared off. And it does get very, very cold, as we shoot all year round. So people do disappear because of the cold, and it gets very quiet, too.”

But in the summer it changes?

“Absolutely, it does. They grab that time, especially the midsummer night, which doesn’t end. It all goes a bit mad – like being in 'The Wicker Man' – and they’re prone to getting their kit off in the winter. Four o’clock in the summer everything stops. Sixty per cent of Swedes have second homes, even little beach huts, so the act of enjoying the summer season is very important to them.”

Henning Mankell, who died last year, encouraged Kenneth to make the final series of 'Wallander'. The two men had known each other for seven years.

“He was a combination of hands-on and at the same time he liked to let us have our heads,” says Kenneth. “He wanted our production to have its own life and for him to be surprised by how things were played. I remember he came in one day and looked anxious. He said, ‘I have the last sentence of the last Wallander book’ – but he had only written seven chapters. He was heading to the last thing. It was like he’d found the riddle of the sands.”

With such dark material at its heart, how did the actor lighten his own mood during filming?

“There was a time in the first series where I would just bury myself,” he admits. “This time more friends came out and my missus was there most of the time, so that in itself made for a more normal evening anyway. But it’s been a bit tough to wash this guy out of your hair.

“There was something about being back that was such a part of my creative life and I made good friends there. But Henning was ill at that point, too, so that was tough. The experience was loaded. The thing was, when I drove away I had a funny feeling that it wouldn’t be the last time. I don’t know any more than that, but a spell has been cast.”

So let’s hope we haven’t the seen the last of this compelling combination.

'Wallander' will be shown on BBC1 later this month.