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Leading man Christian Bale admitted that Ledger had created an "iconic" version of the Joker and the filmmakers would struggle to top it in the next film.
He said: "He was a friend, he was great company, and I miss him dearly. He does a fantastic performance - he's really created an iconic portrayal of the Joker and I think the real job if Chris [Nolan, director] decides to create a third movie will be how do we create a better villain than Heath?"
Nolan stated that he was delighted film fans were getting to see Ledger's last completed role.
"I've known for a long time how great Heath's performance as the Joker is and now other people are getting to share in that," he said. "I'm really excited that they seem to be responding to it in a way he would have intended."
Sir Michael Caine likened the late actor's Joker to a classic screen antagonist, saying: "He's so wonderful in it and he's such an integral part, he's the villain. He's one of the greatest villains you've ever seen. He's up there with Hannibal Lecter."
Gary Oldman reiterated calls to give the Brokeback Mountain star an Oscar nomination.
"I think he will most certainly get a posthumous Oscar nomination," he said. "Peter Finch won in 1977 (for Network), so he could win it. It's as good a work as anyone out there."
Ledger died of an accidental overdose of prescription drugs in January at the age of 28.
