Zhao talked to about 60 AIDS patients before six finally agreed to work on the set, or star in the film. Even so, half of them insisted their faces were covered.

Among the three who did agree to have their faces shown was 12-year-old Hu Zetao, a student at Red Ribbon School, an institute for 16 children with AIDS in Shanxi province. Hu's mother died of AIDS when he was 4. He lives with his father and stepmother. When Gu's crew went to Hu's home, they found the family ate separately from the boy. After they finished the meal, he washed his bowl alone.

The scene was captured in Together. Gu told Hu to recall his experiences of being bullied by people and cry as loudly as he could. He immediately did so and could not stop for many minutes.

Hu's teacher Liu Qian worked on the set, too, taking care of the child. Liu has been HIV-positive for 10 years, after an illegal blood transfusion. To her 16 students, the pretty woman is like a loving mother.

The middle-aged Xia, from Shanghai will not reveal how he got the disease. He was the actors' stand-in to test the lighting.

"If my face can change people's attitudes toward the disease even a bit, then let it be uncovered," he says in the documentary.

Xia's biggest dream is to find a stable job in Shanghai. Presently, he cannot even find a place to get his hair cut, as barbers know he has the disease and refuse his custom.

Xia had to leave the set prematurely because he became ill. Before he left he went to every crew and cast member to say goodbye, including Zhang and Kwok.

"I thank everybody," he says in the documentary, "because nobody here discriminated against me".

After three months of shooting, Hu Zetao's family now eat with him. Liu works at the school, taking care of her children, while Xia is still looking for a job.

Living with HIV-positive people affected the crew. At the beginning of the documentary, one crew member was too scared to open his mouth when he knew he was sitting beside an HIV patient. At the film's end he said he now knows the importance of respect.

Not all were equally courageous, though. Two crew members quit the film when they knew HIV carriers were working with them.

Jiang Wenli and other actors tried to build trust between the team members. Zhang Ziyi's niece and Jiang's children visited the set and played happily with Hu Zetao.

"Respect is not about vain speech, it is action. When our colleagues saw my children play with the patients, they learned something," Jiang says.

Several medical experts stayed with the crew, too, providing knowledge of the disease and preventative measures.

When the documentary wrapped up, director Zhao Liang took an HIV test.

"I did that to tell everybody, it is totally OK to befriend these patients if you know how the disease spreads," he says. "They are not guilty, they need our love and respect."

The 84-minute documentary has been regularly screened for free at Beijing's Broadway Cinematheque since Dec 1, with English subtitles. Life is a Miracle will hit theaters in early 2011.