I believe the author is saying that "The Godfather" was able to capture the subtlety of communication between Sicilians. Meaning how most Sicilians communicated with each other during the time period. The author is saying, "in particular" to show that he's impressed with the way the movie recognized this subtlety of communication among that group especially. In the second part of the sentence, the author is saying that "The Godfather" also captures the essence of how Italian people in general communicated with each other. In other words, how the majority of Italians communicated with one another.