How can the movie The Imaginarium of Doctor Parnus have Heath Ledger in it when he's dead?

The way I understand it, he goes through a mirror into different worlds and he turns into a different-looking guy every time. He's Johnny Depp in one and Jude Law in another... Could be wrong but thats what I heard. Heath did the first part before he died and obviously couldnt finish it so they changed it to this way.