London makes extensive use of physical descriptions to make clear the transformations Buck has undergone. We are 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 is 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.
The vernacular of these men is certainly noticeable: "Dat Buck, him pull like hell. I teach him quick as anything." Now, while we never hear Buck speak (what with him being a dog), we do hear his thoughts through the third person narrator. And, again, although they are not literally the words of Buck, we always get the feeling that he is composed and intelligent. So the men’s language adds to Buck’s nobility and his position (in the mind of the reader) above the men. Then again, it could also just be 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.
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 is an understanding between them that eclipses all the other relationships in the text.