"I had locked this film with Ranbir when 'Barfi' was only one schedule old. I had offered it to him and he liked the idea very much and consented to do it," Basu said.
Ranbir will have to change his look to get into the boots of the stalwart singer.
"We did some look test of young Kishore Kumarji on him (Ranbir). We have gone through several such processes before zeroing in on him to play Kishoreda on screen," Basu said.
The film will bring back the black and white era where Kishore Kumar reigned as a comic star and trace his rise in Bollywood's playback scene.
"It is a biopic so we are not taking the liberty to make our own story. There are a lot of things that people don't know about him," Basu said.
Kishore Kumar's son Amit Kumar is involved at every stage to provide inputs for the film, Basu revealed.
"The family does not have a problem with the film. They have already given the green signal. They are helping us out a lot with his personal things, secrets, which the people are not aware of," he said.
Kishore Kumar had married four times. His first wife was Bengali singer and actress Ruma Ghosh. Their marriage lasted from 1950 to 1958.
He later married actress Madhubala, who had worked with him in many films including his home production 'Chalti Ka Naam Gaadi' (1958) and 'Jhumroo' (1961). They got married when she was sick with a heart ailment. However, the marriage was also strained and lasted nine years, ending with Madhubala's death in 1969.
Later, Kishore married Yogeeta Bali in 1976, but separated in 1978. Then he was married to actress Leena Chandavarkar from 1980 until his death. He has two sons, Amit Kumar (playback singer) with Ruma, and Sumit Kumar with Leena.
Meanwhile, speculations are rife that Katrina Kaif might be signed to play Madhubala in the film.
"We have not thought of anyone as a leading lady yet. As of now only the three of us (Basu, UTV and Ranbir) are final," he said.