Much ado about Wallander

AT 55, Kenneth Branagh is philosophical about middle-age. "You are entering a different kind of territory, where the 'lived in' bit of you might be more interesting, whether that's inside or outside," reasons the five-time Academy Award-nominee.

On the 'outside', Branagh concedes that "gravity is doing its thing to the face, and all the rest of it", while 'inside' he has experienced a few "senior moments".

"I may be delusional, but I've decided my brain is doing exactly what my brain needs me to do," he says with a chuckle. "If I can't remember the thing I can't currently remember, it's because essentially it's low priority. It's cleared that bit of my inbox!"

But middle age is no roadblock when it comes to playing craggy, maverick detective Kurt Wallander, in the hit BBC series Wallander that's been running since 2008. Based on books by the late crime writer Henning Mankell, Wallander centres on the eponymous detective, as he investigates violent and terrifying murders in Skane, southern Sweden.

The fourth – and final – series sees Kurt facing a heartbreaking decline in his health, as dementia sets in.

Branagh, who was born in Belfast in the Sixties but moved with his family to Reading when he was nine, hasn't experienced Alzheimer's first-hand, but in preparation for the role, spoke to friends who had.

"It's a very interesting thing that Mankell puts [dementia] into a detective story, when sometimes the identification of Alzheimer's or dementia is, in itself, a detective story," notes the actor.

"'Where did I put my keys?' becomes, 'I've been in the front hall for 10 minutes not even knowing why I'm in the front hall', and somebody says, 'Did you forget your keys?' Then that person who's coming back into consciousness goes, 'Yes that's right, I forgot my keys'."

As Kurt's health falters, his personal life becomes rosier. "He's happy – in Wallander terms," says Branagh with a laugh. "He has a difficult relationship with his daughter, which seems to be finding much more constructive territory, but particularly, he's utterly charmed by his grand-daughter."

Laughing as he calls himself an "Irish sentimentalist", Branagh admits it felt "unusual" to part ways with the series, and the friends he made in Ystad, where it's filmed. "Ystad is so isolated that it really does become a large part of your life," says the star, who directed Lily James in the 2015 live-action version of Cinderella.

"With the first series, I used to go and bury myself there a bit. This time, more friends came up, my missus was there most of the time, so that in itself made for more normal evenings," he says, referring to wife Lindsay Brunnock (Branagh was previously married to Emma Thompson, and was in a relationship with Helena Bonham Carter in the mid-1990s). "It was a little tough to wash this guy right out of your hair!"

But there are things he – and other cast members – won't miss. "Jesus Christ, it's cold there in winter," Branagh exclaims, breaking into a grin. "I remember one of the first scenes we shot was at the docks at night. It was in April, there was a wind blowing off the Baltic that was just bitter, and a poor fellow had to play a corpse with a T-shirt on. Unfortunately, we didn't get to his close up until four in the morning, at which point we couldn't shoot him because he was rocking with the cold."

The summer came with its own peculiarities, too. "In midsummer, it's suddenly like you're an extra in The Wicker Man – not always very comfortably, I have to say," the actor states with a laugh. They're hard and large drinking people in my experience," he adds, describing the Swedish locals' aptitude for the extreme weather, "and they're prone to getting their kit off in the winter anyway, so it certainly happens in the summer. We get very angsty crew during summer; they want to go to the beach. They weren't interested in Brits working until it was done," Branagh teases, breaking into an impression: "'It's finished? Now we need to go down and be mad on the beach. I have a bottle of a stimulating spirit and the desire to do this before winter comes again!'"

As well as Wallander, Branagh is famed for bringing Shakespeare's soliloquies to the masses, in acclaimed film adaptations of Henry V, Much Ado About Nothing and Hamlet. It was in the 2007 stage production of Othello, starring Chiwetel Ejiofor and Ewan McGregor, that he spotted a young Tom Hiddleston.

"He made Casio so effortlessly charming and was so adept, adroit and invisibly easy with the language. It did feel like the start of something, because this kid, against those two, really stood out'," he recalls.

A year later, Hiddleston appeared in first series of Wallander, and then Branagh cast him as the villainous Loki in his 2011 superhero movie Thor, with the younger man – and potential Bond in the making – acknowledging the director for giving his career a boost. "Humphrey Bogart many years ago said it takes ten years to become a star," says Branagh. "Now I watch The Night Manager ten years on from that moment, and say, 'Hello, here's a star now fully emerged'."

• Wallander, BBC1, Sunday, 9pm