DeNiro shouldn't have played this part anymore than he should have played Bill the Butcher in Gangs of New York. For one, DeNiro couldn't pass as Irish. Also, using Daniel Day-Lewis as an example, it takes away from the shock value. Daniel Day-Lewis playing that role was mind-blowing casting, and it got him a nice Oscar nomination. That said, in The Departed, I would've preferred Bill Murray in the role instead of Nicholson. The actor who plays the role in Infernal Affairs was generally considered more of a comedic actor before that. Bill Murray in the role would be far more interesting than DeNiro or Nicholson. DeNiro, like Nicholson, would just be playing a role he's done before.