There's a reason why Robert DeNiro is considered one of the greatest actors of all time, and it's because when he's really taking a role seriously, there's no one more intense, entertaining, and honest to watch.
If you look at all of his great performances, it comes from that intensity and frustration with people that he has, he doesn't want to put up with people's BS and even when he's doing comedy, it comes from that same scary place.
He might be getting up there in age, but I don't think I'd ever have the balls to say any of those things to DeNiro's face. Also, I wouldn't want to, the man made The Fan a great movie. THE FAN. I rest my case.