Amitabh Bachchan is known to sign on unusual roles and the latest is that the superstar will play his son Abhishek's grandad in Rakeysh Mehra's forthcoming Dilli 6.

Mehra's semi-autobiographical film has Abhishek playing the lead and also stars Sonam Kapoor, who made her debut in Saawariya. He also roped in Amitabh to play a specially written cameo.

Confirming the news, Amitabh said: "I play Abhishek's grandfather in Dilli 6.

Interestingly, the film already has veteran actress Waheeda Rehman. And one of the Big B's incentives for stepping into the picture so late is to share screen space with his favourite actress.

Amitabh has earlier worked with Mehra in Aks. And with Dilli 6 almost complete, Mehra already has plans of casting Amitabh in his next film unusually titled Paanch Kaurav.

"It's too early to talk about Paanch Kaurav. But it's in the caper-adventure genre. And it would be my third film with the Big B. I couldn't get luckier. I've also done three films with Abhishek," chuckled Mehra, whose last film was Rang De Basanti.

"I was supposed to do Abhishek's launch film Samjhauta Express and then we were to do Bhairavi (with all the three Bachchans including Jaya). We had gone so deep into the discussions that we both feel we've already worked in those two films together. So our comfort level in Dilli 6 is as high as it would be for any director-actor team that has done three films together," the director said.