Without either character having said a word, we have a strong opinion about their nature, explain.

Steinbeck spends a lot of the narrative describing both men in Chapter one. He uses animal imagery and tone to illustrate both men. Consider Steinbeck's description of Lennie:

[He] walked heavily, dragging his feet a little, way a bear drags his paws. His arms did not swing at his sides, but hung loosely. ch 1