From Jake Is Huge by Chris Heath, GQ Magazine, May 2010, page 110:
Between courses at the Ravagh Persian Grill in Manhattan....(continues)
To promote the movie, the cast appeared together on Oprah. For the first half of the show, it was just Gyllenhaal and Heath Ledger fending off Winfrey's enthusiasm and curiosity. There is a sheen cast over the movie now--to some extent because of its subject matter but mostly because of Ledger's death--that leaves it and anything connected to it frozen, untouchable. So rewatching the two actors on Oprah, I was surprised to see Gyllenhaal alluding to times that the two of them didn't get on during the filming. Gyllenhaal seems surprised, too, when I mention this, as if he wasn't aware this was something he'd shared. But he remembers. The scene with the two of them by the river, for instance.
"Where Heath's character goes into how his father knew of two guys who lived together and he ended up seeing them killed and dead. I had always read it in a certain way. I heard it in my mind a certain way. And I had worked with these two incredible actors like Dustin Hoffman and Susan Sarandon. When I worked with Dustin, he would always do things to me; in between a take, he would stop his lines, and he would be like, 'You're a wonderful person,' or horrible things sometimes to jar me...getting a different response. And so I decided in this scene to gather the gumption"--this was when Ledger's part of the scene was being filmed and Gyllenhaal was off-camera--"and I don't subscribe to this really, changing lines on an actor, because I don't think I'm Dustin Hoffman, and I don't think I have the ability or the talent to do that to somebody. But I do think that changing intention sometimes on an actor when you're doing a scene when you're not on-camera is really interesting for them. But I did it at one point in this one scene, and I'd always heard it a certain way, so I was almost trying to move the scene to that place for him. He delivered the whole thing very, very straight, and he could feel me trying to do that, and I remember him turning to Ang at the end of it and going, 'If he wants to tell me what to do, have him tell you.'...We had these kind of exchanges, 5 you know. But ultimately, the way I look at it was I was wrong, because he was brilliant. But at the same time I think, "Well, if I hadn't, would it have gone a certain way?" We balanced each other out. When I think about these things that happened then, we were very much alive in that movie. We were really living that movie. Not literally, but you think about those times in your life..."
[5]
One thing Gyllenhaal would do--feeding off the way Ledger had chosen to play the part of Ennis quietly, his teeth clenched--was to ask him in character, "What? What'd you say?"
"Stupid me," Gyllenhaal comments now.