"I think he has done a phenomenal job in playing the 13-year-old. Although intelligent and witty, he is playing a brat in the film. He is not spoiled but he is a brat,"

adman-turned-film director Balakrishnan said.

The 67-year-old star will be essaying the role of a child suffering from progeria, which accelerates ageing.

International make-up artist Stephan Dupuis, who did Robin Williams' make-up in 'Mrs Doubtfire', was roped in to give Bachchan the special look in the film.

With blood veins prominent on his bald head and broken teeth, the actor looks five times his age as the 13-year-old 'Auro'.

"The look is a very interesting one. It's a very authentic reflection of progeria patients and that is why the look works so well," Balakrishnan, popularly known as Balki, said, adding that managing Bachchan's appearance was not a daunting task for him.

"I didn't have to manage him. He managed everything himself," said the 'Cheeni Kum' director, adding that he did not use any gimmicks or special effects in the film.

"I have nowhere done any special effects or gimmicks in the movie to make Bachchan resemble a kid. And he is going to be of exactly his height in the film. We haven't made him smaller. But its because of the camera, he looks smaller when you see him in the film."

In a role reversal of sorts, Abhisekh, the junior Bachchan, plays Big B's father while actress Vidya Balan essays the role of his mother.

"The whole idea was to make a film with Abhisekh playing the father and Amitabh the son. It all began one day when I saw Amitabh pulling his son's leg after Abhisekh said something very mature. It was then when I said I will make a film with Abhisekh as the father of Amitabh," Balki said.

Interestingly, it was Balki's doctor who told him about the existence of progeria.

"I didn't wanted to make a film on a fictional disease where I child grows old. So I had kept the idea on hold till my doctor told me that such a disease actually exists," he said adding that the film is not about the ailment but about the emotional relationship between a father and son."

However, neither the director nor Bachchan himself ever met any progeria patients.

"You are not allowed to meet them as they are a very special case and we didn't wanted to be insensitive. We prepared ourselves by watching videos and documentaries of such patients," Balki said.

So is 'Paa' following the 'Taare Zameen Par' way by highlighting the cause of a disabled child?

"No, there have been many films on diseases before as well. Cancer started in Hindi movies 40 years ago. Amnesia and paralysis also started way back. Today, we find films with an AIDS background. The diseases are just a story matter, its not the film, because we are not making a documentary," he shot back.

Describing film-making as a very lengthy process, Balki, who is also the chairman of ad agency Lintas, said he sacrificed his vacations to shoot for the film.

"I shot the film by taking leaves from my office. Others take holidays but I spent my vacation in making the film," he said.

When asked about his future projects, he said he hasn't given it a thought.

"I don't know about the future. I have no idea at all," he said.