The surprise of last year’s Cannes Film Festival, Toni Erdmann doesn’t sound like an appetising prospect on paper. A two-and-three-quarter-hour German comedy about a strained father-daughter relationship, set primarily in Romania, it’s hardly what you might call the feel-good film of the year. Even the poster – a blonde lady, face obscured, hugging a mass of black hair (what is it – a gorilla or a mammoth?) was unsettling and weird.

Yet writer-director Maren Ade’s movie became the toast of the festival. “Before, everybody was not hoping for much because of the length [of the film],” Ade tells The Independent in a boardroom in London’s Mayfair hotel. Since Cannes, it’s swept the European film awards, winning all five categories in which it was nominated, including Best Picture. It has been Oscar-nominated for best Foreign Language Film. In Germany, it’s been a huge hit – with more than 800,000 admissions.

The 162-minute running time, it seems, has not been a barrier to success. “There were recently a lot of long films,” says Ade, whose previous two films were The Forest for the Trees and Everyone Else. “I don’t compare myself to Tarantino but he always makes long films and people go. It’s not about length; it’s more about whether it’s a good film. Also with TV series, we’re so used to consuming three episodes and then time has gone on, and if you get your arse off to the cinema, then it’s OK when it’s double-length!”

The 'Toni Erdmann' director and writer, Ade, based the comedy partly on her own father

At a loss to explain the film’s success is its 38-year-old star Sandra Hüller (Requiem, Amour Fou). “I have no idea why it’s like this,” she tells me. “I’m just very happy that people let themselves be touched by our work.” It lost out to Paul Verhoeven’s Elle for the Best Foreign Film at the Golden Globes, but it competes in the same category at the upcoming Baftas and Oscars. In the US, Sony Pictures Classics released the film – in a territory not known for embracing subtitled movies. “It’s up to them,” laughs Ade. “I didn’t tell them to buy it!”

While the film evokes tremendous laughter from audiences, Ade isn’t even bothered whether it’s classed as a comedy. “I thought it worked and I didn’t care what it was – that was the good thing,” she says. “I didn’t care what it would be in the end, whether it would be a comedy, or a drama. I started with a wish to do a comedy and I had the feeling it was not a pure comedy. It was more that I had this character who plays a comedy for his daughter and this is something he is doing out of desperation.”

The film deals with an ageing divorcee, music teacher Winfried (Peter Simonischek), who decides to reconnect with his daughter Ines (Hüller) after his dog dies. Fond of the odd practical joke, Winfried arrives in Bucharest, where Ines is a management consultant at an oil company, to find her barely receptive to his presence. And then the fun really starts as he dons a silly wig and false teeth, introducing himself to her unwitting friends as life-coach ‘Toni Erdmann’.

Partly inspired by her own father – although Ade adds, “We have a different relationship – he never did something like Toni Erdmann” – the filmmaker was intrigued by the notions of identity and adopting personas. She cites the work of the late comedian Andy Kaufman, in particular his obnoxious Vegas lounge-singer Tony Clifton. “On one side, he was this nice, shy guy and then he had this alter ego who did what he wanted. So that was an inspiration.”

Hüller and Simonischek play daughter and father in the film

Then there’s the uptight Ines – a woman climbing a very male-dominated corporate ladder. “She’s a self-determined woman,” says Ade. Even when her boss requests that she takes his wife shopping, her acceptance of this humiliating stroke of male chauvinism is a deliberate ploy. “She knows [that it’s sexist],” accepts Hüller, “but she also knows, ‘OK, I’m in this business and that’s the way it is.’ She’s very unsentimental about these things. She even shares the humour of the men.”

While Toni Erdmann might read as a corporate critique meets father-daughter reunion, set to the backdrop of an ever-evolving Europe, the second half morphs into something even stranger. Take Hüller’s karaoke version of Whitney Huston’s ‘The Greatest Love of All’, a rendition that caused the Cannes audience to burst out into spontaneous applause. Or the appearance of the Kukeri, a nine-foot tall Bulgarian folk creature used to scare away evil spirits. “This Kukeri costume, it’s made out of goat, so it smelled!” laughs Hüller. “It smells terribly!”

Then there’s what has been dubbed “the funniest nude scene of the year” – as Ines, hosting a birthday brunch, cracks and sheds her clothes moments before her colleagues arrives. So was she concerned about performing the scene? Hüller shakes her head. “I don’t want to give the impression of someone who likes to undress in public! That’s not my goal!” she says. “[But this] un-sexual nakedness…I really like this. It’s not seen so often in movies; normally when people are undressed, they’re having sex.”

Still, filming it wasn’t easy, says Ade, who had to gain the trust of her cast for a sequence that masterfully escalates in its hilarity. “It’s the worst thing, just standing naked in a light room. Not even playing a sex scene – at least there you have something to do!” She even offered Hüller the chance for the whole crew to be naked in sympathy during the shooting of the scene, but Hüller refused. “I hate that,” she says. “Who cares? I’m playing this scene, but I don’t expect everyone to cry when I have to cry in a scene.”

While the possibility of a Hollywood English-language remake remains, Ade and Hüller aren’t getting overexcited about any possible forays to the US – apart from a trip to the Oscars, of course. “I really need to have some life in between my films,” says Ade, who has two boys aged five and one. Likewise, Hüller has a young daughter, Ruby. Both feel rather spent after Toni Erdmann. “I feel like ‘How can I do this again?’” says Hüller. “We put everything out that there was and now it’s empty.”