Gotta agree with Neo here. Jack's characters in "One Flew Over the Cookoo's Nest", "Five Easy Pieces", "Easy Rider", and "Witches of Eastwick", "As Good as it Gets" and "Departed" are all exactly the same character despite being in vastly different types of films. It's just him sneering and rolling his eyes for two hours. "This time he's a sneering, smartassy, gangster". "This time he's a sneering, smartassy, writer". He mostly blows.
But I do love his Joker. It's the only time he was cast correctly. No disrespect to other good Joker portrayals, but at least Jack joked around and laughed a bit in his.