"For '3 Idiots', Aamir wasn't the first choice, but for 'PK' he was. I had made Aamir listen to the basic idea of this film during '3 Idiots' itself," Hirani said here.

The ace director has worked with actors as celebrated as Aamir, Sanjay Dutt, Vidya Balan and Kareena Kapoor.

Asked why he always works with stars, he said: "I wanted to work with newcomers in '3 Idiots' and I had even found three people. But while each was good in isolation, together they lacked chemistry. Then Aamir called and said, 'I can play the character' and we changed the casting.

Hirani's 'PK' features Aamir and Sanjay, along with younger actors Anushka Sharma and Sushant Singh Rajput. Sushant has a cameo in the movie. Talking about him, Hirani said: "We wanted a known face for it, but no one was able to give 15 days for it. We thought of casting a newcomer for this small role and went searching for one, but didn't find anyone.''

"My casting director asked me to see Sushant. We liked his audition and signed him. That's when he told me that he was also doing 'Kai Po Che'." "PK" releases on December 19.''

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