"He was wonderful. He was talented beyond belief," said Christian Bale, who plays Batman in the movie. "His immersion in the character was a greatly satisfying thing to work opposite."
"I was in awe of him," said actor Aaron Eckhart, who plays district attorney Harvey Dent a.k.a. Two-Face. "He was very impressive the time that we worked together. As another actor, that doesn't happen all the time. When you're seeing it and you're with it, it's pretty special and you don't want to take that for granted."
Maggie Gyllenhaal plays attorney Rachel Dawes was also inspired by Ledger's performance.
"He did something that is difficult even for every great actor," said Gyllenhaal. "It's still very unusual to hit a stride that is so free that you can really do no wrong. There's no way to judge what he did. He was absolutely free."
"It's him and his talent, I believe in him and I believed he could do this," said director Christopher Nolan. "Because I had seen his talent in other roles, like his character in 'Brokeback Mountain.' That is an incredible performance. With a complete immersion in that character and a complete lack of vanity in the way he plays it."
"When he was in makeup and clothes, he was the Joker," said Bale. "When he took it off, he was great company. He's created a really iconic villain."
Ledger died in his New York City apartment Jan. 22 at the age of 28. The New York City medical examiner said that Ledger died of an accidental overdose of prescription drugs that included painkillers, sleeping pills and anti-anxiety medication.
The cast and director of "The Dark Knight" think that the movie will serve as a fitting tribute to his talent.
"We'll honor him in his performance and by talking about him," said Eckhart.
"I would love to be right here with him. And I thought I would be," said Bale.
"If you're going to go out that young then, you know, go out and do this and go," said Nolan. "A good way to go, in a blaze of glory."