Amitabh Bachchan enjoys experimenting with his roles

"I am happy that I am getting strong character roles. When I was younger I was the leading man for which you have to look good... woo a leading lady... there is a villain. Now because of my age I can't do these roles. Not being the leading man anymore gives me freedom to experiment with different roles and looks," Bachchan said in an interview.

The 67-year-old actor plays Auro in 'Paa', a 13-year-old child suffering from progeria, which accelerates ageing.

Bachchan has also done the role of 'genie' in film 'Aladin' and a ghost in 'Bhootnath.

"It is very challenging to be offered these kind of different roles. For an artist every day is a challenge and it is good to try out different genres. Earlier, it was said that you are doing the same kind of films and now I am told that there is a lot of experimentation," he said.

'Teen Patti' has Bachchan playing Venkat, a Mathematics professor, 'Rann' will see him as Vijay Harshvardhan Malik, a media baron and 'Johnny Mastana' has Big B as John Pereira, an elderly man on a journey of self discovery and penance.

But the most challenging role in the actor's four decade long career seems to be enacting the role of a child, where his father's role is played by none other than his real life son Abhishek Bachchan.

"It was a challenge to get into the make up and playing the role of a child. The make up made life easier while playing the character," Bachchan said.

The veteran actor had to undergo a five-hour-long make-up process to get the look of the character.

"I had my grand children and other kids of that age to observe how they  behave in a particular manner," he said.

Bachchan said Balki made him go through medical journals and discuss the progeria syndrome with various doctors.

"We found a lot of material on how children affected by this ailment look like and what happens to their physique. We finalised a face and worked on how a person of my age would look like that," the actor said.

Christian, an Oscar winning Hollywood makeup artist who has worked on films like 'Lord of the Rings' was roped in to change Bachchan's face.

"A cast was prepared and the face was made through prosthetics. We took liberties with the height. I believe that people who come to see my film are aware of my height and if I were to shorten myself it would have looked impractical and not too genuine," Bachchan said.

"We shot from a particular angle, used different lenses to make me look a little short when I was with Abhishek. Also, the teeth are false and I changed my voice to sound like a 13-year-old, hopefully," the actor said.

'Paa' produced by AB Corp and Big Pictures has been made at Rs 15 crore budget and will release on December 4.

"Make up comprised a major chunk of the budget, but it is creditable that we completed the film within the set financial target," he added.