You can generally tell how well versed one is in a foreign language based on how fast they speak it. If you notice, most Italians who speak Italian first, talk very quickly to each other--so fast so that you would have difficulty understanding them even if you knew some italian. That's what I remember about my Italian relatives. DeNiro definitely spoke it faster than Pacino. The one who seemed to speak it the fastest was Young Clemenza and I don't even think he is really Italian--I think he might be Jewish or something. Also Genco was pretty proficient in it.

~~ Lollie