One, not-often appreciated reason for the film' success are the performances of Scheider, Shaw and Dreyfus, who wor brilliantly at creating believable characters, each of whom have their own understandable reasons for going on this mission other than stopping the carnage. Although the character of Quint is made into a bit too much of a mythic figure, the characters of Brody and Hooper are instantly believable and likeable and become easy to root for. It's a real shame that Scheider's recent health problems, including his battle with cancer, have seriously curtailed his recent output because he's a great actor who projects a terrific cerebral intensity. This also has one of my very favorite final dialogue exchanges of any movie, and I'm surprised it's not quoted more.

A personal note: I was born in 1975, and a few months before that, my father, at the urging of my mother, grew a beard so that he would "look more like a dad". People immediately started noting his resemblance to Dreyfus in this film, and he liked the reaction so much, that he's kept it ever since.