I'm sitting opposite director Eric Toledano in a hotel next to the Louvre, getting the lowdown on how Rahim was invited to join the cast. "He became really well known after A Prophet," says Toledano with a throatily elegant Parisian accent, "especially because he won a Cesar Award for Best Actor and the film was nominated for an Oscar." Rahim's reputation was as an actor who kept everything quiet, but when Toledano met Rahim in the flesh, he realised there was more to the young man. "Yes he acts 'with his eyes' as we say in France, but in real life he was very light-hearted, verbal, sunny and always joking," reveals Toledano. "I asked him if he was interested in playing comedy, in joining the cast of Samba. He didn't look convinced at all and said 'you want me to do comedy in front of Omar Sy!" Toledano laughs at the memory. Sy – one of France's best known actors has an uncanny ability to make people laugh and was better known as one half of the TV comedy duo Omar and Fred before turning to the big screen.

But getting Rahim to play against type was only part of a daring casting process. The two writer-directors decided that, given their success with The Intouchables, they could be bold. With only an idea for the film, they flew to Los Angeles to talk to Charlotte Gainsbourg. "It's so different setting up meetings in L.A.," admits Toledano with a smile. "You can just organise to have a chat, well before the film is ready. In Europe, if you have no script, they'll just tell you to go back home to your mother. So we organised the meeting in L.A. and said to Charlotte that we were thinking of making a comedy for her. She was really happy with the idea and laughed that it would be a change from working with Lars von Trier!" Toledano was thrilled, clearly a fan of Gainsbourg's work. "She's very good in everything – even as a singer, so it was great that she accepted the challenge of working with us – her character in the film is very different to most things she's played – very shy. Shy but with buttons that can be pushed."

Samba starts with a scene in a Parisian nightclub, with a long Steadicam shot reminiscent of Martin Scorsese's famous Copacabana scene in Goodfellas (Toledano acknowledges the homage with a knowing grin). After dancing girls, a wedding cake, and music that could be from the 1930s we move slowly back through the club, into the kitchen, where we find our hero, Samba Cisse (Omar Sy) washing dishes. He's a migrant trying to do the right thing, working his way through menial jobs, saving for a better future. Gainsbourg plays Alice, a depressive, burnt-out executive who meets Samba at a detainment centre, and much of the film – social-drama meets rom-com – follows their relationship amongst the reality of working life as an immigrant. Toledano emphasises the point: "for me the movie is about work. It begins with a guy working in the kitchen and ends with two people going back to work. And everything in between is about work. This is the world of the film." He pauses for a moment and leans in, "You know we humans are scared of not working and yet we suffer most from working too much."

The conversation moves inevitably to the political – to French attitudes to immigration. "It's a very touchy subject in France," says Toledano. "Politicians are always speaking about it, and we wanted to examine some of the unspoken reality of being a migrant in the film – the fact that no one else wants to do the work that these people do. At an economic level, we need them because it's less expensive, but they have no rights, and life is very difficult for them. It's this kind of existence that we wanted for the film – but with comic, Kafka-like situations." And with talent like Gainsbourg, Sy and Rahim involved, it's a highly watchable exploration of both the bitter and the sweet.

Samba opens on Thursday, April 2.