Heath Ledger's most important role was as The Joker in Christopher Nolan's The Dark Knight, but it wasn't his last. His last role was in the inventive and original Gilliam picture, The Imaginarium of Dr. Parnasuss. He actually passed away in the middle of filming, leaving the film in a tricky situation. Terry Gilliam decided that instead of reshooting everything, he would keep Ledger's footage and bring in different actors to play him in "different worlds." While most films couldn't get away with this type of casting, the story actually led itself perfectly to it. When I saw it in theaters, if I had not know the circumstances around filming, I would have no idea that it was intended to be cast any other way.
Another interesting thing to note is that Gilliam was very selective on who would step into Ledger's shoes. He wanted to keep it to Ledger's personal friends, so it would serve as an homage of sorts. The three men also gave their payment from this role to Matilda, Heath Ledger and Michelle Williams' daughter, so her financial future would be secure.
All these lead to a different viewing of the film than if it were viewed in a vacuum, if you had no knowledge of the Heath Ledger tragedy.
Also of note, Andrew Garfield plays the eager young assistant. This was the first I'd seen him, but he's since blown up with roles in The Social Network, Never Let Me Go, and will be the new Spider-Man. I enjoyed him in this role, but never thought he'd become so well known so quickly!
The film itself is a unique viewing experience. With the dreamscapes in the Imaginarium, I'm simultaneously in love with it while wishing it had a bigger budget to make them even more fantastical. There were no limits on what they could look like, no limits on what they could do in there. If you can, try to stream this to your television through a game console or an HDMI cable hookup. It definitely needs more than a small laptop screen to be fully enjoyed.