Khan's decision to potentially quit had partly been propelled by Indian director Mira Nair editing him out of her 1988 hit Salaam Bombay! Though she did make up by casting him as one of the leads in 2007's The Namesake, which remains his favourite film.
The Lunchbox centres around the Mumbai system of dabbawallahs, a community of illiterate couriers who for 120 years have picked up meals cooked by housewives and deliver them across the city's crowded transport network to their husbands' crammed offices.
The story follows Khan's Saajan, a lonely widower who mistakenly receives the wrong lunchbox from a young isolated housewife Ila (Nimrat Kaur), whose cooking is superb. They start a clandestine correspondence even if she doesn't realize he's about to retire - Khan is playing 20 years older in the film.
"I hated that", he smiles "but I believed in the story so I'd do anything. I've been looking for love stories. I'm passionate about love stories and I don't have many chances to do them in cinema. If I get offered Genghis Khan I don't want to do that."
Khan modelled Saajan on his uncle, a one-time clerk in Bombay. "He used to get up at 6:30 and take a bus and train to reach his office. I still remember the faces of people when they'd come back from the office. They looked like somebody had sucked their blood. They looked dead, as if no energy was left in them. That image is stuck in my mind."
But the film is also a celebration of Indian cuisine, something Khan is passionate about.
"Yes!" he agrees with a smile. "I like to keep it very simple and home-made, mostly vegetarian or the way my mother used to cook : Muglai food. It's a middle-eastern kind of preparation using mutton in different ways, like pillau, nihari and passandas. I told Ritesh they should add passandas when my co-worker Shaikh [Nawazuddin Siddiqui] tries to lure me for a home-cooked meal, because passandas are something you never get in restaurants.
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"For me the basic consideration about eating food is I want to smell the vegetables when I'm eating; I need to feel the smell of the vegetables." He goes on to explain at length his aversion to "smelly fish".
How does he feel about the erosion of Indian traditions?
"Tradition us eroding everywhere, all over the world," he replies. "That said India, with its mix of religions is unique. Change is bound to happen. You cannot control that, but you can only tell a story about it."
What are the traditions that he doesn't want to see changed? "The connection between people, the connection with your relatives and parents and the romanticism of each other's bodies in movies which has become very mechanical. The romanticism of letter writing, the romanticism of writing love letters, that's worth preserving."
The Lunchbox is screening now at selected cinemas.
