In the original storylines based on the original script (like the book), Keys deprograms the dog to hate White people instead of Blacks. Once Samuel Fuller came on board, he quickly changed this storyline to simply rid the dog of its racist overtures.
Just before the White Dog finally takes the hamburger from his trainer, he looks up at him, and just under his lip, shows the edge of the prosthetics that hold his cheeks in a snarl.
(at around 11 mins) While at the dinner table, platter of vegetables is across table from girl, then next to her at closeup, then back across the table again at next cut.