Well, here's what Michael Caine had to say about Ledger...

"Heath Ledger stunned me" continues Caine. "Jack played The Joker as sort of a benign nasty clown - like a wicked uncle. Heath plays him like an absolutely maniacal murderous psychopath. You have never seen anything like it in your life. He is very, very scary. I turn up very month or so and do a couple of bits, then go back to London. I had to do this bit where Batman and I watch a video which The Joker sends to threaten us. So i'd never seen him, and then he came up on the television in the first rehearsal and i completely forgot my lines. I flipped, because it was so stunning, it was quite amazing. Wait until you see it, its incredible".