Slumdog Millionaire launched Dev Patel's career in 2008. Besides making him a star, Danny Boyle's Oscar-winning international hit, filmed on location in Mumbai,  also transformed the British actor's life in ways that are becoming clear to him only now.

Patel was 17 when he shot the film. He was a lifelong Londoner who knew very little about his parents' Indian heritage.

Before his visit to Mumbai to shoot the movie, Patel had been to India only once. "I went back as a kid to a family wedding in Madras, but I was too young really to remember," Patel told me during a recent visit to Philadelphia to promote his new drama, Lion, also filmed in India.

"What I do remember is that I got bitten by mosquitoes, none of the kids there would talk to me, and I didn't have my Game Boy, so I hated it.

"But then I experienced it with Danny Boyle," Patel said, "and that was very different."

That experience ignited a cultural and cinematic love affair that continues between Patel and the subcontinent. Patel, now 26,  has returned to India again and again, to shoot films including The Second Best Exotic Marigold Hotel, The Man Who Knew Infinity, and now Lion, costarring Nicole Kidman, Rooney Mara, and David Wenham, and opening Christmas Day.

Lion is the remarkable true story of a 20-something Indian Australian professional named Saroo Brierley who used Google Earth to track down his birth family in the city of Khandwa, a family and a city he had last seen when he was 5 years old.

Based on Brierley's best-selling 2014 memoir, A Long Way Home, the film is neatly separated into two parts.

The first recounts how 5-year-old Saroo (played by newcomer Sunny Pawar), who was born into abject poverty, became separated from his family after falling asleep on a train. By the time the train stopped, the boy was nearly 1,500 miles from home. He was eventually adopted by an Australian couple.

The second half, set two decades later,  shows how Brierley, now played by Patel, becomes obsessed with finding with his birth mother.

Patel said he pounced on the role as soon as he heard about the film.

"I had read an article about [Brierley], and I was blown away. I was just really baffled and moved by his story, and I called my agent, who said there was a script in the works," Patel told me during a chat at the Logan Philadelphia hotel.

"You just don't get such an emotionally nuanced character, with this depth," he said. "Normally, I get these very quirky, peaky supporting roles."

But there was more to it than the chance to land a great role. Patel felt a deep affinity for Brierley. He knew what it felt like to have an identity crisis based on one's ethnic origins.

He understood why Brierley "had spent his life trying to suppress some part of  himself. … He would say to his friends at college, 'Oh, I'm not really Indian.'"

As the child of immigrants, Patel had gone through a similar struggle.

"His journey is similar to my own in a way because growing up in London and going to school there – and I want to a pretty rough school –  I tried to suppress that heritage, too," the actor said.

"I did everything I could to shun that heritage and fit in a much as I could and not stand out and be bullied."

Patel said he'd like to continue making films about India.

He's shooting Hotel Mumbai, a thriller about the series of coordinated terrorist attacks at the Taj Mahal Palace hotel in 2008 that led to 164 deaths over four days.

Patel is also an executive producer on the film, which costars Armie Hammer and Jason Isaacs. He took on the added responsibility, he said, because he wanted to ensure the film would not be exploitative.

"It's about the real heroes of that day, the staff, who taped baking trays to their chests and ran out in front of these men with AK-47s to save the guests," Patel said.

"This is my fifth film about India. and I've loved [working on] it. ...  It's the land of such juxtaposition. There's such a history, but also so much technology and modernity and a youth that's just now finding its voice.

"There's such a diversity of humanity all around you," Patel said.  "It's the place to explore what it means to be human."