Yung Chang's documentary Up the Yangtze, playing in the World Cinema Documentary Competition, is imbued with such intimate, soul-bearing scenes that after seeing it you're left wondering, "How did he capture that stuff on film?" Like when a proud young man gets fired from his job, or when a poor mother tearfully admits to exploiting her 16-year-old daughter to support their family-they are moments that could only have been revealed to a filmmaker who had established a deep trust with his protagonists.

"Time is essential for getting to know your subjects," said the Chinese-Canadian filmmaker. "Before bringing cameras onto a location you have to build that trust, that friendship, because it goes beyond turning on the camera and making them do things."

Chang followed two young people for the film-19-year-old Chen Bo Yu and 16-year-old Yu Shui-as they left home to work on one of the many "Farewell Cruises" that operate on China's Yangtze River; so-called because they offer tourists a last-chance glimpse of the river culture before the Chinese government begins flooding the area to make way for the colossal Three Gorges Dam, thereby displacing the millions of people who now live alongside its banks.

Chang first learned about the tours in 2002 from his parents, who invited him and his grandfather on one such cruise. He described the scene when he embarked on the ship in Chongqing-the world's largest municipality- as a vision straight out of Blade Runner. "We arrived at night and there were coolies running everywhere, grabbing our bags and carrying them down," he said. "They appeared in silhouette because it was dark and everything was lit up with neon lights because they light up the river in pink and purple and blue... It looked really apocalyptic. And as we marched down the steps toward the boat this drum roll began... There was a marching band playing "Yankee Doodle Dandy" and "You Are My Sunshine." At that moment I decided it had to be made into a film."

He returned in 2003 and spent two years researching, looking for characters, and building relationships with them. He found Chen Bo Yu and Yu Shui-or Jerry and Cindy as they were known on the ships-while tagging along on a recruitment trip with a company called Victoria Cruises. "All these kids [from the river towns] would come and interview with the cruise line. It was like this natural casting," Chang said. "I had some stipulations, I needed a girl and a boy and they had to be from families that were going to be displaced by the flooding. There were a few... but it ended up the best stories were Jerry and Cindy."

But sometimes the best stories for filmmaking are also the most painful in real life. Both children and their families went through tremendous emotional stress throughout that time in their lives. Chang was deeply affected. "I'm going through a kind of post-traumatic stress right now," he admitted. "Seeing these emotional things is really tough. The scene where the mother is crying and talking about exploiting her daughter-we were all in tears at that moment. You can even see the camera shaking. It was heart-wrenching. And then you go home at night and as a director you have to plan what you're going to do next. You have to bury those deep feelings to get through it."