He finally zeroed in for Farhan Akhtar to play the part in the movie, penned by the National award-winning Prasoon Joshi.
"It took us two and a half years to write the script of 'Bhaag Milkha Bhaag'. Prasoon Joshi has written the screenplay. But once the script was ready, the biggest question was who will play Milkha Singh? It gave me a lot of sleepless nights," Mehra said on Tuesday.
"I could not sleep at nights due to the tension that who will play this role," he recounted on the set of singing reality show 'Indian Idol Junior', where he was present with Farhan to promote the movie.
'Bhaag Milkha Bhaag' talks about Milkha's struggle, his perseverance as an athlete and also his courageous human spirit.
Mehra said he wanted such an actor for the role, who has his own interpretation for the character.
"I would not say this role is difficult but with the kind of films that are made in our cinema, it's a very different role. It requires more of the interpretation of an actor," Mehra, known for films like 'Aks', 'Rang De Basanti' and 'Delhi 6', said.
"We were not making an exact copy of Milkha Singh, and there shouldn't have been mimicry and all that. Suddenly one morning Farhan's name came to my mind. I immediately messaged him. Then we met, spoke and this is how it was done," he added.
Releasing July 12, 'Bhaag Milkha Bhaag' features Sonam Kapoor opposite Farhan.
Photos: Katrina Kaif, Shamita Shetty at Mumbai airport
Photos: Varun Dhawan, Alia Bhatt promote 'Badri...' on 'Dil Hai Hindustani'
In pictures: 15 facts about kissing that will surprise you
Photos: Shahid Kapoor and Mira Rajput at cafe in Bandra
Photos: Deepika Padukone, Neha Dhupia and Soha Ali Khan at event