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.

u00a0

"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,u00a0said 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.