Well yes, yes and yes! Johnny Depp, Jude Law and Colin Farrell signed in.
To understand why and how, I give you the plot of the movie according to Wikipedia:
In the present day, immortal 1,000-year-old Doctor Parnassus (Christopher Plummer) leads a traveling theater troupe that offers audience members a chance to go beyond reality through a magical mirror in his possession.Hmmm.. sounds interesting.
Parnassus had been able to guide the imagination of others through a deal with the Devil (Tom Waits), who now comes to collect on the arrangement, targeting the doctor's daughter (Lily Cole).
The troupe, who is joined by a mysterious outsider named Tony (Ledger), embark through parallel worlds to rescue the girl.
The character played by Ledger is reportedly to be transformed magically into additional incarnations of himself as portrayed by Johnny Depp, Jude Law, and Colin Farrell.