Good interview here: http://thecriticalcritics.com/interviews/interview-elizabeth-healey/
"Because the film was to be totally improvised the rehearsal was a bit different from any other film I’ve been involved in. Warren, Keir and I talked through together in detail what the storyline was and what was happening at any point in the timeline of the film. We then walked the route, through London that we would be taking during filming, so we had a good idea of where we would be. It also allowed us to include any ideas into the film that came to mind as we explored the route."