While film director Rajkumar Hirani has admitted that Ranbir Kapoor is the only actor he has spoken to about playing the lead role in a biopic on Sanjay Dutt, the actor says it will be an immensely challenging part to essay.Talks have been on about Ranbir getting cast for Sanjay's role in the movie. But the actor, who has shown his versatility through films like "Barfi!" and "Yeh Jawaani Hai Deewani", told IANS: "Biopics are hard because you have to celebrate such a giant man's life on screen... I would really look forward to getting Sanjay Dutt alive on screen. "It's a very challenging character, very colourful... he's had a very challenging life and I think it will be interesting to portray him on screen."However, Ranbir clarified that since Hirani is still writing the script, nothing is finalised yet.Hirani, in the meantime, has said that he will "starting the film next year".