We begin with a chat about the last scene of the Dardenne brothers' new film. Nobody is likely to confuse Two Days, One Night with a Miss Marple mystery, but we will, nonetheless, draw a veil over the details of the denouement. Suffice to say the beautifully crafted neo-realist fable does not end in any sort of joyous cavalcade.

Starring Marion Cotillard as a an ordinary woman threatened with the sack, the picture treads in similar territory to early classics by the Belgian directors such as Rosetta and The Son. It is, however, a little bit less miserable. Non?

“Yes, maybe,” Jean-Pierre says. “A friend of mine watched the end and he says: ‘It is a film where the main character loses her job and it is, for the Dardennes, a happy film.’ Ha ha!”

Luc creases up and joins in with his brother’s chortles. The translator has a laugh. I’m hooting. Come on in. It’s fun time with the Dardenne brothers.

This is not the first time it has been said. But little in the output of Jean-Pierre (63, a little broader of face) and Luc (60, square around the temples) would prepare you for the experience of meeting the brothers. Over the past 20 years, since they belatedly found rhythm with The Promise, they have perfected a school of hard naturalism that, though indebted to Ken Loach and the Italian neo-realists, is very much their own. Ordinary lives take in extraordinary tragedies while a mobile camera slopes behind the actors' shoulders. They are the great sombre humanists of the age.

Yet the two men could hardly be jollier.

“My brother and I talk about this a lot,” Jean-Pierre says. “We have a friend, an actor, who makes everybody laugh the whole time. Then when he is on stage suddenly he is so serious. It all falls away. We are funny in real life? I think so, yes. But when working on a character, we get serious. Why is that? We don’t know? If you have an explanation, then tell us.”

Mind you, over the past two films, some shift in tone does appear to have been setting in. To that point, when watching the brothers' films, the viewer was always aware that catastrophe was lurking at the protagonist's elbow. The Kid with a Bike and Two Days, One Night allow in just a little more light.

“Oh, we are getting older,” Jean-Pierre says. “Maybe the films are getting more optimistic. That must be it. In this film somebody does say: ‘I am happy’. So that’s maybe true.”

The Dardennes took a circuitous route to their current renown. Born and raised in Liege, Neither had any great ambition to be a film-maker as a boy.

“No, Jean-Pierre wanted to be an archaeologist and I wanted to be a mechanic,” Luc says. “But those things are linked with what we do, maybe. One is about storytelling the other is about construction. So it’s maybe not so surprising.”

As things worked out, Jean-Pierre ended up studying drama and Luc took a degree in philosophy. Some years after their graduation, the playwright, poet and film-maker Armand Gatti – a man they describe as a “spiritual father” – brought them together to work on one of his theatrical productions. They began shooting bits of film and video. By 1975, the brothers had formed their own production company and were making documentaries on a range of subjects.

Modest acclaim In the early 1980s, Gatti lured them to Northern Ireland to help out with his little- seen film Nous étions tous des nom d'arbres. Here is a project that cries out for rediscovery.

“We were in Derry and the countryside during the hunger strike,” Luc recalls. “It was hard to get permission to shoot. But Armand had been in the SAS. So, he persuaded them.”

For more than 20 years, Jean-Pierre and Luc plied their trade with only modest acclaim. They made two dramatic features, Falsch (1987) and Je pense à vous (1992) – but they admit that neither reflected their very original sensibilities. The Promise, released in 1996, found them hitting their stride. Then, three years later, to the surprise of many, Rosetta – the story of a teenage girl and her alcoholic mother – took the Palme d'Or at the Cannes Film Festival.

Beating the likes of David Lynch, Jim Jarmusch and Pedro Almodóvar to the top prize, the Dardennes, then both closing in on 50, found themselves belatedly and suddenly elevated to the pantheon. When l'Enfant took the Palme in 2005, they joined a select band of six (now seven) film-makers to have grabbed the prize on two occasions. Nobody has won a third.

Most everybody thought that Two Days, One Night would pick up an award this year. But, for the first time, they won not a single prize

“Oh, that is all part of the game,” Jean-Pierre says. “We are not in films for that. We don’t think about that. We don’t have the money enough to pay the jury! Ha ha! No, I’m joking. We are always happy just to be included.”

As well as an apparent lightening of mood, the two most recent Dardenne films have seen them employing movie stars for the first time. The Kid with a Bike features Cécile De France as a kindly woman who takes care of a wayward youth. Two Days, One Night hangs around one of France's most recognisable actors. The directors admit that moving towards more familiar faces was a conscious choice. They were interested in playing with preconceptions.

"We produced Rust and Bone with Marion and we were already working on a character," Luc says. "We wondered if we could bring a well-known face into our world. We met Marion in the flesh and we had a cinematic love for her. Ha ha! But it was another character then. It was a young doctor. We couldn't get it to work. Then Sandra, a character we'd been working on for years, popped around the corner and said: 'Hello! I'm here!' We then thought about Marion for that instead."

That’s interesting. So, the brothers will allow characters to mill around their psyches for years before they actually find a place in their films?

“As we get older, we have more stories behind us,” Jean-Pierre laughs.

The scenario for Two Days, One Night is an arresting one. Set in an ordinary corner of Belgium, the film finds Cotillard playing an ordinary woman who, shortly after recovering from a period of depression, finds herself being laid off in unusual circumstances. If she can persuade a majority of her colleagues to forswear their annual bonuses then she can keep her job. A weekend of cajoling and pleading follows.

Do such things really happen?

“The starting point was real,” Luc says. “This person lost their job because she didn’t have the support of her colleagues. That made us think. The idea of somebody being pushed out by her employees was an interesting one.”

Is there less solidarity among workers than in earlier times? Certainly, on this side of the English Channel, the unions are not nearly so strong as they once were.

“That does seem to be true. People are not so secure in their jobs as they used to be. They have trouble making ends meet. You know, it’s easier to be supportive when you’re secure. It’s not the unemployed who are being asked to show solidarity.”

The new film is a delightful and moving addition to one of the strongest canons in world cinema. Over the past two decades, Luc and Jean-Pierre have quietly allowed legendary status to fall on their shoulders. I certainly believe them when they say they are not in it for the prizes. But, now tied with the likes of Michael Haneke and Francis Ford Coppola on two Palmes d'Or, some part of them must long to be the first to grab a third.

“No, no, no,” Luc chuckles. “Look, our favourite football team is Standard Liège. So we are used to losing.”

Football teaches you about life?

“It does, it does, it does.”

More chuckling. More shoulder-heaving. They really are the most agreeable interviewees in the business.