The dialogue helps us feel what these characters are feeling. Each character is angry in their own way: the father is angry at his son for getting in trouble at school, the mother and son are angry that the father isn’t taking the son's side.
Notice we don’t need to be told exactly what the characters are feeling (as in, “Son, I am very suspicious of you and that’s making me angry!”) in order to understand them. We can understand their feelings through details in the dialogue and action.