"This is not a historical or a biopic in the sense that he was born here, he lived here and he married here. What really intrigued me was his undying spirit," Mehra told in an interview.
"This boy, who came from a village and was an orphan at 11, actually witnessed the massacre of his family members, including his brothers and sisters. He picked a knife to survive at 11. He spent time in (Delhi's) Tihar Jail before he joined the army.
"He wanted respect and to be a human being against all odds. That's what a wonderful human being he is, and that's what got me into the movie, not the records he made. 'Bhaag Milkha Bhaag' is not about the athlete, it is about the spirit, and its interpretation," he added.
Mehra admits it has been a challenging affair - his "most challenging" as yet.
Milkha Singh, who once held the national record for the 200 metres and 400 metres, is best remembered for his fourth place finish in the 400 metres at the 1960 Rome Olympics.
He won India's first international golds in 1958 at the Commonwealth Games (Cardiff) and Asian Games (Tokyo) - and a gold at the 1962 Asian Games (Jakarta). During his career, he won 77 of the 80 races he ran. So, will the film be inspiring for the audiences?
"No" said Mehra.
"Films are not meant to inspire. It is a happy accident if it inspires - at best! How do you make an inspiring film, I don't know!"
The biopic features actor Farhan Akhtar in the lead role and is slated for a July release. Milkha Singh agreed to share his life's story with Mehra for Re.1 - and gave Mehra much more than the filmmaker ever thought.
"He has been very kind. He (almost) adopted me for two years; so I lived in Chandigarh. I got to know him better and entered the dark corners which he had kept close even to his dear ones, children and wife. Prasoon Joshi wrote the script. I feel incredibly lucky to speak to the person about himself," said the filmmaker.
"It was like making a film on Mahatma Gandhi and speaking to him. Though Milkha Singh is not as universal as Gandhi, he is definitely Bharat Ratna material, I would say," the 49-year-old added.
Mehra, the brainchild behind films like "Rang De Basanti" and "Delhi-6", also said he was happy to get a free hand from his co-producers Viacom18 Motion Pictures to mount the film on the scale he wanted to - in a "so-called high budget," he said.
"I could have easily spent five times more, especially when I was making a film showing the Olympics and the Commonwealth Games! I had to rely on creativity to express it all in a budget, but having said that, in the current Indian scenario where budgets stand, I have been very lucky," he said.
"My co-producers Viacom have not just given me resources in the current market situation, they re-assesed and increased the resources by 30 percent when they saw the film's rushes. They said, 'We never thought this is the way you would mount it'. So it is fantastic to see that change is happening," he added.
