It's the moment in the film where Chief Brody really realises the gravity of the situation he's in. In the moments leading up to these scenes he is elated when the fisherman have brought in a shark - which he is of course convinced is the killer. And then he is confronted by the woman whose son was killed.
The minute she speaks - you can see the guilt in Brody's eyes. He doesn't have to say a word. In fact, his face barely moves -- but you can feel the weight of the situation and the guilt he is carrying.
"Cause I need it."
In the next scene he is having a personal, touching moment with his son. He barely says a word. He feels responsible for the death of the little boy - yet here is he now with his safe and healthy son next to him. Again, Scheider barely does a thing; it's all in the eyes.
When Hooper (Richard Dreyfuss) turns up, Brody is just quietly amused. The occasional wry, painful smile. As Hooper talks to Brody's wife he just sits there between them, mulling a million things over in his head.
He pours himself a large glass of wine before giving a small glass to his wife and to Hooper. Up until this point, he's hardly said a thing. As Ellen, his wife, tries to think of the word for his fear of water; Brody jumps in with "drowning," before going in to a very specific question about sharks, which he directs at Hooper.
In these few minutes alone we see Brody's character and purpose completely change. It's a change that informs the rest of the film. Suddenly, the man who has been afraid of water all his life takes to the water without complaint. He has a job to do, it's the only way he's going to beat the guilt that's building up inside of him.
Incredible filmmaking from Spielberg, with heartbreaking music from John Williams. But most of all - Roy Scheider lays himself bare in these scenes-- what he does with his eyes and only minimal dialogue is incredible.