Tools of Characterization
London makes extensive use of physical descriptions to make clear what a transformation Buck undergoes. We're frequently told of Buck’s size, his strength, his muscles, and his body’s hardness. This, of course, reflects the other, non-physical changes Buck has undergone in adapting to the wilderness, which makes it a good tool of characterization.
The man in the red sweater is defined by his actions—beating Buck means he's a cruel man. Thornton, of course, comes into the picture by saving Buck’s life; he is a man of compassion. Spitz, of course, is also defined by his actions (stealing, fighting) and therefore painted as a cruel dog.
Although he is quite clearly a dog, we start thinking of Buck as person. After all, he has thoughts, emotions, and a complex, human-like personality. Buck is characterized by this complexity. By taking on the importance and dimensionality of a human, Buck becomes a real protagonist despite his animal form.
Speech and Dialogue
Broken English: François and Perrault
The way these men speak is certainly noticeable:
"Dat Buck, him pull like hell. I teach him quick as anything." (2.6)
Now, while we never hear Buck speak (what with him being a dog and all), we do hear his thoughts through the third person narrator. And, again, although they're not literally the words of Buck, we always get the feeling that he's eloquent and poetic. So the men’s plain language adds to Buck’s nobility and his position (in the mind of the reader) above the men.
But you can't look at François and Perrault's language without noticing the vernacular they use—this is a device that demonstrates the place and time of the story, highlighting that these are French Canadian men who speak English as a second language and are of the rough outdoors.
Non-Verbal Communication between Buck and Thornton
For a dog that doesn’t speak, Buck certainly communicates with Thornton a good deal. You’ve got your typical dog stuff like hand-licking and tail-wagging, but it runs deeper than that with these two. When Buck wins that bet for Thornton by pulling the thousand-pound load, they seem to speak to each other before and after it happens. At the very least, there's an understanding between them that eclipses all the other relationships in the text.