I think that the development of the characters in the film is where Cameron is able to both sustain the audience's attention and make a statement about the historical event of the sinking of the vessel. On one level, Cameron is able to show how Rose changed as a character because of her affair with Jack. The need to repudiate social conventions when they are wrong and stand up for one's own voice, as well as what one considers right become critical elements that were dormant in Rose prior to her interactions with Jack. Through her relationship with him, these elements become dominant forces. They also become the galvanizing forces that enable her to live when so many perish. Cameron makes the argument that Jack inspires Rose to continue to persevere and struggle through the cold and through the harrowing conditions in order to be rescued. From this, he is able to make the argument that if more people had absorbed these lessons, the tragedy could have been prevented. The captain, listening to his own voice and authenticating his own experiences, would have rebuffed Ismay's notion to proceed full speed ahead, thereby increasing the chance of hitting the iceberg. Had Mr. Andrews rebuked social convention and advocated for what he knew to be right, more lifeboats would have been made available, thereby saving more lives. Had there not been such strict social stratification, enabling the first class passengers to escape more quickly, there could have been more lives saved and less doomed.
Cameron develops the characters, specifically Rose and Jack, in this context. He is able to compose a love story narrative where both are able to appreciate one another more fully because they fully grasp that social conventions that are predicated upon exclusion and marginalization are not social conventions that are worth embracing. In this, he develops both characters, as well as the side characters, showing how these individuals might have benefited from accepting some of these lessons.