After appearing in the TV series Skins, aged 16, Dev Patel, has not looked back. He starred in the Oscar-winning Slumdog Millionaire, The Second Best Marigold Hotel (as a super-keen hotel manager) and The Man Who Knew Infinity (as the great Indian mathematician Srinivasa Ramanujan). Now he stars in Lion, based on the true story of Saroo Brierley, an Indian street boy who got lost 1,000 miles from home, was put in an orphanage and sent to Australia for adoption. Twenty-five years on, Saroo started to search for his mother, using Google maps.

Congratulations on your Golden Globe nomination for Lion. Is Saroo the most demanding role you’ve played?

I’m 26 and, like most actors my age, hungry to show emotional range. This role enabled me to play a character suffering real pain, a change from the scripts that want you as a funny sidekick. It took eight months to prepare. I wanted to commit every fibre of my being to getting it right. I had to bulk up, grow my hair, learn the accent. At my last audition, where I met the director Garth Davis, I’d been in The Man Who Knew Infinity and was skinny, with a buzzcut. I had to get myself a personal trainer and started eating like a glutton – downing the protein shakes.

I never went to acting school. Everything I’ve learned, I’ve learned from great directors and my co-stars

It is a movie about mothers and sons. What did your mother make of it?

I brought my mum, father, sister and grandparents to the London film festival premiere in Leicester Square. It was a beautiful, full-circle moment. Ten years previously I’d stood outside that cinema at the Hancock premiere waiting, for three hours, for Will Smith. I got a picture of his forehead and remember thinking: wow, this is so incredible. Now my goofy mug is on these pictures and I’m standing next to Nicole Kidman. My mum was very proud, and torn to pieces by the film.

Wasn’t your mother responsible for getting you into acting in the first place?

She is a big inspiration. She is a social butterfly, jovial, a real character. I’m far more introverted. She is the one made to go on screen, not me. The reason I’m in the industry is because of her. She saw an advert for the Skins audition in Metro, tore it out and dragged me along to the National Youth Theatre in London. I had to bunk off school.

You had lots of energy as a boy. Do you still?

It’s different now – I’d call it drive. When I was younger I was hyperactive. I did martial arts. When my friends saw me at the martial arts academy, they’d say: “This is a different Dev: you’re so calm and disciplined.” That is a big part of acting too. Part of the prep with Garth was trying to find stillness. Saroo is split in two – the child [Sunny Pawar] reacts to a harsh city and uses street smartness to survive. The Australian adult is more inward, battling demons, trawling through pixels on his laptop, trying to find his mother.

How did the real Saroo contribute?

He is a generous, confident young man. We’ve become close friends. He told me that when he first came to Australia as a child, he’d go to bed, his heart would race and he’d feel himself coming out of his body, hovering over India. He felt himself materialise on the streets, he’d find his mother and brother and tell them he was OK. He’d do that every night and wake in the morning sweating and exhausted.

Do you keep discovering new aspects of India with each film you make there?

India is a constant source of inspiration. I’m on my fifth film – The Hotel Mumbai – about the attacks on the Taj hotel in 2008, another Australian/Indian co-production. For Lion, I travelled across India on trains and felt the isolation – partly because of not speaking the language. At each stop I’d hear a different dialect as the landscape slowly changed…

You used to worry that the roles on offer for you were limited – do you still?

Yes and no. There should not be any limitation to playing my culture. I’m a British Asian, it is part of the fabric of who I am. My grandparents are from India and Nairobi. So what I’m trying to say is that Lion and Marigold and The Man Who Knew Infinity are completely different. Journalists sometimes label them as “Indian guys” as if this were an umbrella term.

What have you learned from working with great actors such as Judi Dench, Maggie Smith and Nicole Kidman? And what do they have in common?

They have a curiosity about life, a sense of humour and emotional reserve. I never went to acting school. Everything I’ve learned, I’ve learned from great directors and my co-stars. Acting is about honesty. When I began, I was trying to squeeze as much emotion out of roles as I could and get big laughs. Now it’s about doing less, cutting away the fat.

Did Google advise on Lion? And are you technical – your clapped-out BlackBerry has been mentioned in interviews…

Google helped us by digging out the 2008 version of Google Earth from their archives… and, no, I’m not technical. I’ve updated my BlackBerry only because I needed Uber. I have an iPhone but a very old one.

Do you ever feel your life is a dream?

All the time. I’m just this guy from Rayners Lane, how the hell did this happen? I’ve a friend, Sam, who tells me a story about perspective: think about living even two doors down from where you started – imagine how drastically different your life would have been. I’m lucky to have incredible parents. It takes a lot to let your son go at 16 to chase his dream. I’m excited to be bringing them to LA for the first time to stay in my new house.

Facebook Twitter Pinterest Dev Patel in Slumdog Millionaire. Photograph: FoxSearch/Everett / Rex Features

Nicole Kidman on Lion and adoption: 'It's about the simplicity of love' Read more

What projects of your own are you working on?

I’m writing a hyper-modern action film, based on 5,000-year-old Hindu mythology, set in a heightened Bombay – an anthem for youth.

Do you ever stop working?

I’m about to catch a plane to Australia and have 22 hours peace. Saroo and his family will be there for the Sydney premiere.

Do Saroo and his family approve of the film?

Our producer told me that they rented a private theatre to see it. Saroo sat in one corner to experience the film alone. Sue and John, the mother and father, were in the middle, holding hands. Mantosh [his adoptive brother] was in the other corner. When the lights came up, they all came together and were holding each other’s hands and crying. I thought that was beautiful.

Lion opens on Friday