Hotel Mumbai is a blow-by-blow account of the siege of the Taj hotel in 2008, when a group of 10 terrorists launched simultaneous attacks on several high-profile targets in India's biggest city. Four gunmen invaded the hotel, a fabulously luxurious palace where, as the head chef tells his staff in the opening scenes, the guest is God. Patel plays a Sikh waiter called Arjun. We first see him with his wife and daughter at home. Home is a domestic nest, full of love. It is also a slum, a dank couple of rooms in a labyrinth of similar rooms. On the fateful day, Arjun is nearly sacked from his job because he has left his one pair of proper shoes behind. Fortunately, for the people his cool head and determination will help save over the course of the next three days, he isn't. Dev Patel as Sikh waiter Arjun in Hotel Mumbai. When the hotel staff realise the building is under attack and the exits blocked, they go into a collective crisis mode. First of all, the guests must somehow be made safe. The most secure room in the hotel is a plush private bar and billiard room that has more than a whiff of an Agatha Christie murder mystery about it. As do many of the guests: the hard-nosed Russian oligarch played by Jason Isaac, the American heir to fortune with a patrician Indian wife (Armie Hammer and Nazanin Boniadi), the overbearing white woman teetering on the edge of hysteria (the late Carmen Duncan). The Christie setting brings with it a sense memory of the fear that a killer could be in the room – which, of course, could be true any moment here, too. It is one of many tricks and tropes of genre Maras uses to keep tension simmering.

"They attacked many parts of Mumbai," says Patel. "But why this is particularly interesting is that the hotel itself is a character; it is a microcosm of India. At the bottom you have these waiters who work in the kitchens and basements and live in poverty in these tiny chicken-coop homes, but when they put on the magical clothes that are their uniforms, they are all of a sudden allowed to stand next to a Russian billionaire and pour him blue-label vodka that costs more than one of their annual incomes." Patel with Hotel Mumbai co-stars Armie Hammer and Tilda Cobham-Hervey at the film's Australian premiere at the Adelaide Film Festival in October. Credit:David Mariuz That class gulf is a central subject of the film for Patel; it is also deeply disquieting. "I remember watching a documentary about the Taj that Anthony sent us, where you saw a hand cutting the grass with scissors to make sure it was all level," he says. "And then you step out into the city and there is such a need for proper infrastructure for whole communities, proper sewage systems." For those within and without the temple walls, however, the Taj's grandeur was a source of pride: it was the first building in Mumbai to be wired for electricity, played host to rock stars and politicians, was as good as anything in the world. Even one of the terrorists, in a line taken from the real attackers' recorded conversations with their command base, describes it as paradise. "That for me is mind-blowing, you know," he says. "That idea that a place at the Taj was an ideal and what the attack meant to Indians." Like all but one of the characters in the film, Arjun is a composite of the imaginary and several real people. "It was a conscious decision from the start how we dealt with the true-life stories," says Maras. "Chef Hemant Oberoi was a public figure prior to the attacks and I had done many interviews with him, so I felt comfortable with representing him on screen. For the others, who were private people before going in, it was a conscious choice out of respect to conceal their identities. But pretty much everything you see in the film happened to somebody. There was a waiter who helped shepherd people out of the hotel, but here he is combined with another person who joined forces with the police."

Patel's waiter Arjun lives with his family in a Mumbai slum. Credit:null It was Patel's idea to make his character a Sikh, marked out by his distinctive turban, because he thought it would add a twist to the theme of religious conflict. "I read lots of accounts of Sikh taxi drivers who were beaten up after 9/11, with people calling them all sorts of racial slurs." Clearly, there was a widespread ignorance about who Sikhs were; here was an opportunity to educate people. He didn't know much about Indian culture before he made Slumdog Millionaire, which was also his introduction to the Taj Mumbai: the big dance sequence he still says he hated doing was filmed there. After that came The Best Exotic Marigold Hotel, where he played the eager young hotelier, then The Man Who Knew Infinity, about pioneer Indian mathematician Srinivasa Ramanujan. The richness of the country really revealed itself to him, however, when he was travelling around by train to prepare for Lion, soaking up its multiple dialects and visiting orphanages. His parents are Gujarati-speaking Indians who came to Britain from Nigeria, but he was very much a Londoner. "Growing up, the idea is to do as much as possible to fit into that place and rid yourself of all cultural and ancestral ties," he says. "Being introduced to India by Danny Boyle blew my mind. It was so much more than I imagined." Patel with Freida Pinto in the Oscar-winning Slumdog Millionaire.

Arjun is restrained, inward and intense – very different from the ebullient Patel – and very afraid. "It's weird, the opposite of acting really, trying to add humility and fear so that you are trembling head to foot even during acts of heroism," he says. "It's sort of anti the Hollywood cliche, the person on the billboards. Bruce Willis is going to get battered and bruised and bloody, but he's going to survive." Terrorists are not so selective; as Maras likes to say, bullets don't discriminate. Maras comes from Adelaide, where much of Hotel Mumbai was shot; he comes to film having studied law and history, with a particular interest in justice issues and what seems to be a dogged commitment to research. He devoted a considerable amount of effort to finding out who the terrorists were. The Indian security forces had intercepted and recorded the terrorists' satellite contact with their handlers in Pakistan, so that he could listen to what they said and what they were doing from one minute to the next. He also read thousands of pages of court transcript from the trial of the one terrorist who survived the attacks. "We built the story of Imran, a character in our film, on this guy's confession and what came out during the court case," he says."That opened my eyes to another side of the story. When you see how it was that they were recruited, you realise it's a much bigger story than just these 10 guys." Patel as Saroo Brierly in Australian film Lion. All came from remote areas and knew so little of the world they had never even heard of the flush toilets they discovered in the Taj. "They went through different tactical exercises but also a lot of indoctrination. One of the points I hope comes across in the film is that it is the extremist ideology driving them, not those 10 individuals, that is the danger."

What inspired him to make the film, however – a crazy logistical undertaking, he agrees, for a first-timer – was the way staff and guests in the hotel rose to that terrible occasion. "It really is a story, as I see it, about people from all walks of life coming together to survive a pretty horrific ordeal," he says. "You had people from all socio-economic backgrounds, all races and religions; all these divisions that are so prevalent in society came crashing down." In crisis, agrees Patel, everyone was suddenly equal. "You could see the true humanity of the people in that place. A lot of people that died in the Taj were the staff. You can read the accounts of them strapping on baking trays and pots and pans, and running in front of AK47 fire to save their guests and their fellow workers." Patel (second from left) and the cast of Skins, his breakthrough TV show. Credit:Everett Collection Inc / Alamy Stock Photo When he began acting, Patel worried in public that there would only be a limited range of roles available to him. As it turns out, along with making five films in India, he has also been cast against convention as David Copperfield in Armando Ianucci´s forthcoming adaptation of the Dickens classic and as medieval knight Sir Gawain in experimental American director David Lowery's next film. He can do anything, in other words. And the Indian roles are not entirely plain sailing: he is aware of some undercurrent of resentment that an English actor has become the face of India for international audiences. On the other hand, a film such as The Man Who Knew Infinity, which tells such a remarkable story of Indian genius, could only be greenlit because the Slumdog guy was in it. Now he has been able to help launch Hotel Mumbai. "A film like this, with a bunch of brown faces, would not have been made 10 years ago," he says. "But people are embracing new faces. Why not embrace brown ones?"

Hotel Mumbai opens on March 14