The Rakeysh Omprakash Mehra-movie is based on the life of athlete Mikha Singh.
"I did the film only for Rakeysh Omprakash Mehra. It's a special appearance, but I hate the word special appearance. It's a cameo. I did this film because I knew for sure this film will go down in history," said Sonam.
The actress, who won accolades for his performance in her last release "Raanjhanaa", says she is choosy about her film projects.
"I'm choosy about my films. I do one film in one to one-and-a-half years. My films take too long - like 'Mausam' and 'Sawaariya'," she added.
"Bhaag Milkha Bhaag" stars Farhan Akhtar in the lead, and it releases Friday.