Image copyright Parkland Image caption David Kross, who plays Bert Trautmann, always wanted to be a footballer

Bert Trautmann was born in Germany but he went on to have one of the least likely careers in British football. The former prisoner of war from Bremen became one of the most acclaimed British goalkeepers of his generation, playing eventually for Manchester City. Now the film The Keeper tells his story.

David Kross is 28. As a teenager in Germany, his first big film was Knallhart (aka Tough Enough). Then in 2008 his role in The Reader, opposite Kate Winslet, brought international fame.

But as a child, what he really wanted to be was a professional football player. "I always loved the game," he says. "From five years old until I was 15, I was totally sure I'd be a footballer. It was the same with most of my friends but for me, it didn't work out. So I became an actor."

In The Keeper, he's been called upon to revive his skills on the pitch. It's the true story of Bernd Trautmann, born in 1923, who was in the German forces in World War Two. In 1944 Trautmann was captured by British troops and ultimately sent to a prisoner of war camp near Wigan.

Image copyright Parkland Image caption Freya Mavor as Bert Trautmann's wife Margaret provides the emotional drama

Somewhere along the way, the name Bernd became Bert.

His talent as a goalkeeper registered and he ended up playing for local side St Helens Town in the old Lancashire Combination League. In 1949 he moved to Manchester City as a professional and stayed until 1964, turning out for the club more than 500 times. He died in 2013.

The film is a German-British co-production and in Germany it's simply called Trautmann. But director Marcus Rosenmüller admits that few Germans under 65 would know who the central character is.

"But that's not a big problem because the story really has to work as a love story and a family drama. It's not just a sports bio-pic, although of course the football scenes have to be convincing."

A lot of the hard work of giving the story emotional depth rests with Freya Mavor, who made her name playing Mini McGuinness in the final seasons of Skins on E4. She plays Trautmann's first wife Margaret.

Image copyright Parkland Image caption David Kross was able to draw on the football skills he learnt when younger

Mavor can't claim to compete with her co-star's passion for football but says: "I've lived quite a lot in France, so when France won the World Cup last summer, I did go a bit mad."

From the ages of nine to 13 Mavor lived in La Rochelle on France's Atlantic coast; at 19 she moved to Paris. Her fluency in the language means she's been in several French films. "I've always loved French cinema and I was obsessed with not sounding like a tourist. So it's brilliant to be accepted as an honorary French person in film."

But was it hard to produce a convincing accent for post-war industrial Lancashire?

"The big test was to sound authentic for people in St Helens and Manchester now," Mavor says. "But I also wanted to understand the society which Margaret came from historically. There's a fascinating book by Norman Longmate called How We Lived Then. It was a huge help in understanding what the war was like for most people."

Image copyright Parkland Image caption Margaret (Freya Mavor) supported Trautmann despite fears over his past

At this point Kross confesses that, though much of the film is set there, he never actually went to St Helens.

"A lot of The Keeper was filmed in Northern Ireland and our football ground was in Belfast. But that's how films work. Later, when you see me at Wembley with Man City, that's mainly CGI: we were actually filming at Augsburg in Bavaria."

It was in the 1956 FA Cup final at Wembley, when Manchester City were playing Birmingham, that Trautmann damaged vertebrae in his neck. Despite being in a fog of pain, Trautmann played on to the end of the match, securing his side's victory. It was only later he realised how severe his injuries were.

The film starts with a short but powerful section in which we see Trautmann fighting in World War Two. Kross says those scenes were essential. "We have to understand the times he grew up in and the criminal regime which dominated Germany. Bert was part of the Hitler Youth and he went through a sort of brainwashing.

"He absolutely wanted to be a soldier. But there are interviews Bert did near the end of his life in which he talks about seeing civilians shot in Ukraine and how that changed him."

Image copyright Parkland Image caption Freya Mavor and David Kross filmed much of The Keeper in Northern Ireland

Kross says The Keeper is basically about a man seeking a new home. "I think that's the emotional centre and that's what I needed to get right as an actor."

In the last part of his life, Trautmann lived in Spain and it was there that director Rosenmüller went to talk to him, several years before filming began.

"We spent a week talking to him and as I sat there, I wondered why no one had filmed his story already. There is such drama in how Margaret accepts him and then how his teammates accept him and then England accepts this man they thought was a Nazi."

Rosenmüller always knew there would be a German release for the film but he resisted the temptation to reshoot Kross's scenes in German.

"Visually the German and English versions are 98% the same and David was in the odd position of dubbing himself into his own language. Almost all the German is spoken early on and in fact that helps the drama - the audience sees that Bert is lost in a world he doesn't understand."

Kross comes from Schleswig-Holstein, near the Danish border, but he now lives in Berlin. For a year he was at drama school in London but he's worked in German- and English-language films. So where does he now see the centre of his acting career?

"I would love to do more British-German co-productions. But that doesn't really happen much: it has to be a story which will interest both audiences and the film industry doesn't very often come up with these stories."

The rate of change in that industry is clear when Kross and Mavor talk about what they're doing now. His next project is Betongold (Concrete Gold) - one of the first Netflix productions in Germany.

Mavor, meanwhile, has been filming a four-part fantasy in French called Il était une seconde fois (Once Upon Another Time). It, too, is to be seen on Netflix.

The Keeper opens in UK cinemas on 5 April.

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