This year's Doc/Fest in Sheffield, which I caught the tail end of at the weekend, had a great selection of films on show. Of note were Men of the City, Marc Isaac's surprisingly tender portrait of workers in the City of London, and Erik Grandini's Videocracy, which unpicks the poisonous links between celebrity culture and political power in Berlusconi's Italy. There were also arts docs covering, variously, Brian Eno and the making of Tarkovsky's Stalker.

For me, though, the real star was Once Upon a Time Proletarian, by the Chinese novelist and director Xiaolu Guo. It consists of interviews with 12 people drawn from across Chinese society: from an ageing peasant who longs for the days of Mao, to frustrated small-town teenagers, to rich Beijing businessmen. Filmed in spare moments during the shooting of Guo's forthcoming fiction film, She, a Chinese (which I wrote about here), it has a wonderful spontaneity that captures a fleeting moment in China's history.

Below, taken from the Q+A session that followed the screening -- and abridged by me -- Guo explains what her film has to say about modern China.

I wanted to make a group portrait presenting the different social classes in China that have arisen over the last 20 years. China used to be a very simple society: before communism it was made up of peasants or slaves and the emperor system. From the 1950s onwards the whole country changed, but it was still a simple social structure: soldiers, workers and peasants.

Now China is in this period of radical change, where people are losing their identities, losing faith in both traditional and socialist values. There's a new middle class, millionaires, even billionaires. People who used to have a peasant identity, who spent their lives planting rice, now have to work in factories making computer parts.

[The film] shows a kind of collective melancholy. This whole country has lost its old values, and we were never a religious country, so now everyone has to force themselves to be civilised.

I don't trust objective narrative. For me, the truth is subjective and emotional. I don't want to know people's life stories, all I want to know is that moment when they look at the camera. It's very powerful but it's over so quickly.