That was really interesting!! I wish I could see similar footage of Heath getting into the Ennis character. In this one he seems to be trying to create the sort of "spastic" energy that the Joker has throughout the movie. And, for Ennis I can imagine it would have been a really difficult effort at trying to become as mellow or introverted as possible and to really reign in any excess energy to become sort of tightly-wound in Ennis's way.
I know Heath was superb as the joker, but I can't help but feel it was maybe an easier part for him.He naturally had so much barely contained energy,I am not surprised when he said it was the most fun he had, or probably would ever have playing a part.It probably seemed like a massive relief ,just to let it all out and run with it.
The role of Ennis is so counter Heaths' personality, with retrospect, it must have been an extraordinarily difficult part for him. To have to continually rein in all that manic energy which he had in abundance.
The more I think about it, the more I feel it is still his defining role.So I am in full agreement with you.I suspect for Heath even small things like keeping his hands still, as Ennis must have been a constant battle.Probably the only scene which was like Heaths natural personality type, is when he is pacing around, up and down drinking, waiting for Jacks arrival.