The Dark Knight director Christopher Nolan has written a wonderful tribute to his late star Heath Ledger in Newsweek titled “Charisma as Natural as Gravity”. You can read it here, and it will show you what Heath was like as an actor — definitely quite amazing.
Also, Daniel Day-Lewis dedicated his Screen Actors Guild award for Outstanding Male Performance in There Will Be Blood to Heath, who he had never met. You can watch what he said here.
Source: Superhero Hype!