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It's well known that when you let actors improvise, you have to start with a fixed idea and direct the improvisation in a certain direction, otherwise it doesn't work. Nothing happens - apart from using up loads of film. You have to sow the seeds. You create characters out of dust and blow life into them. You have to have some sort of plan, a plan that you impose on the actors, whether they're conscious of it or not. Then they can carry on working on the characters that you've given them.

Lars von Trier
from Trier on von Trier