The focal point, to me, was the ushering in of slavery to the foreground. In the novel, that topic was delicately, but firmly, positioned into the background. The movie found a way to both address it and even use it as a catalyst for some key pieces of action.
Tom’s alcoholism, in the movie, became a symptom of his anguish, suffered due to the mental chore of reconciling immorality with desire. Being the older son, he was, of course, due all the wealth that his father had acquired, along with all the accoutrements of riches and power; however, the wealth was built on a foundation of the inhuman practice of slavery. Tom’s excess in drinking led to his serious injury, which was the impetus for much of the social-maneuvering and jockeying by the Crawfords, at the center of the story.
Moreover, the sketchbook, which Tom composed – and which probably sparked his cognitive dissonance and fueled his misery – displayed to Fanny the very extent of immorality. Fanny was appalled by these images and rightfully blamed Sir Thomas, but the sketches were so shocking that the audience could not have moved past. And since the director’s vision seemed to hinged on the connection of Fanny (a character that was seemingly disconnected in the novel) to the audience, it was necessary to disallow the glazing over of such depictions of violence and rape. This connection in reproach, between the audience and Fanny, led to Sir Thomas severing his ties of Antigua and picking up the leaf.