The shooting of the film, which is being directed by Terry Gilliam, suspended due to Ledger's death by accidental overdose on Jan.22. The decision to use three actors to replace Ledger was made possible because of the fantastical nature of the story, which involves a magical mirror that takes people into different dimensions, allowing Gilliam to switch between actors.
Before his death, Ledger had completed shooting the 'realistic' sections, so Depp, Farrell and Law will play physically transformed versions of his character when he passes into parallel worlds of the imagination.
"Since the format of the story allows for the preservation of his entire performance, at no point will Heath's work be modified or altered through the use of digital technology," Variety quoted producers William Vince, Amy Gilliam and Samuel Hadida, as saying.
"Each of the parts played by Johnny, Colin and Jude is representative of the many aspects of the character that Heath was playing," they added. Gilliam said: "I am grateful to Johnny, Colin and Jude for coming onboard, and to everyone else who has made it possible for us to finish the film."