Hirani, who has worked with Dutt in "Munna Bhai" series, will be unfolding the 56-year-old actor's life on the big screen.
"I have worked with him (Dutt) in three films. I am not saying I am friends with him...but I know him. I respect him and he respects me and so possibly he chose me to narrate this story," Hirani told PTI.
"Last time when he was here, I narrated him the whole script... he is aware about what is happening," he said. Hirani was fascinated with the real life story of the "Khalnayak" actor.
"For me the only reason to make a film is when I hear an engaging story and nothing else. I was fascinated by his story, I dropped everything away and thought this is the story I want to do," he said.
Dutt's life has been with lots of ups and downs -- from his drug addiction days to his involvement in 1993 Mumbai serial blasts case and his personal life.
"In any biopic...of two-and-half to three hours of the film you cannot show entire life of a person. We are selective in showing what the world wants to hear about him. Like in 'Gandhi My Father' film, they chose to show his personal life and not the struggle," the "3 Idiots" helmer said.
The biopic, to be helmed by Hirani, will have Ranbir Kapoor essaying the part of Dutt.
"It is at a very early stage...one draft of the script is done. And we have signed Ranbir Kapoor. But beyond that nothing has been done," he said.
In real life, Dutt has married thrice. Since the movie is being made on his life, there might be more than one actress in the biopic.
"We are still working on the script...there will be changes in the script. We are yet to take a call on that (heroines)," Hirani said.
When asked if Dutt might co-produce the film, Hirani said the logistics are yet to be finalised.
The film will go on floors this year and is likely to release around Christmas, next year.
