I've heard George Clooney say they enjoyed pulling jokes on each other during Batman and Robin. As far as praising each other? I don't really remember it. But it wouldn't surprise me if they did, because from my perspective, they really did seem to enjoy working with one another and loved playing jokes on one another.
As far as Michael Caine's quote? People seem to think he's saying this because the studio is forcing him. Michael Caine is a legend and a wonderful actor, he doesn't seem like the type to cave in to studio pressure and praise something for the sake of praising it. I remember during the Daniel Craig controversy Judi Dench said "You'll see, he's a wonderful actor and he does the character justice."
Not only was she right, but Craig's performance silenced the naysayers and Casino Royale was an amazing film.
Caine obviously see's something in Ledger for him to be singing high praises about his performance.
Most actors and directors praise films that they're working in because they have confidence in the material. Elijah Wood, Sean Astin, Peter Jackson, and Viggo Mortensen all said "Return of the King is much better than The Two Towers", and guess what? They were right in my opinion. All of the films were excellent and masterpieces, but Return of the King really brought the story together and LOTR is the trilogies of all trilogies in my opinion.
You've also had people who aren't part of the film and have seen Ledger in the make-up say he looks downright creepy and looks completely in character.