New Delhi: The idea to cast Amitabh Bachchan as Abhishek's son in "Paa" struck filmmaker R. Balakrishnan when he watched the father-son duo together and observed a high level of maturity in the latter's behaviour.

"My main aim was not to make a film on progeria. My main motive behind making 'Paa' was to reverse the roles of Amitabh and Abhishek. I decided to do something like this because once I was with both of them and I saw Abhishek behaving in a very matured manner. It was then that this idea struck me," Balakrishnan, who is known as Balki, told IANS over phone from Mumbai

The director revealed that he got to know about progeria, the premature ageing disease, much later and used it to make his film look real instead of a fantasy fare.

"I didn't know how to implement my idea. Then a few friends introduced me to this ailment called progeria. I consulted doctors and even researched on the internet to know more about it. This gave me the idea of making my film look believable," he said.

"Paa", an emotional story of a father-son relationship, will see Amitabh playing Abhishek's 13-year-old son Auro who is suffering from progeria.

The much-awaited film is slated to release Dec 4 and has been made under Amitabh's banner AB Corps Ltd.

Balki, who had cast Amitabh in his directorial debut "Cheeni Kum", says he was convinced that Amitabh was right for the role in "Paa" right from the inception of the idea.

"I was convinced from the very beginning that Amitabh will be best suited for this role. He has given brilliant performances before, but this is his most mind blowing performance," he said.

"For the role, he had to be completely different from his real self. For an actor to give a performance away from his personality is a very difficult thing," he added.

For the special look of Amitabh's character Auro, Balki roped in noted international make-up artist Stephan Dupuis and the arduous make-up procedure used to take more than four hours.

What is unusual about the promos of "Paa" is the fact that Amitabh has been referred to as a newcomer.

Asked about it, Balki said: "We wanted to remove every bit of Amitabh Bachchan from the character and present him as a completely new person. His distinct style, mannerisms and the way he talks...we wanted to remove all that. That was the whole purpose of this exercise."

Balki is also impressed with Abhishek and Vidya Balan, who play Amitabh's parents in the film.

"Abhishek is a very sensitive, understated and subtle actor. As an actor and as a son you need courage to play father to your father in a film. It's extremely difficult but he has done a great job," he said.

"As far as Vidya is concerned, I believe that she is the finest actress we have today. She can play mature roles as well as exhibit childish qualities in her," he said.