We watched the film director Mike Leigh get interviewed by Mark Lawson a few weeks ago, and it was a fascinating insight into his working methods. I've seen quite a few Mike Leigh films, the most recent of which was Happy-Go-Lucky, and I always admire the way he gets such great performances from actors. His modus operandi is famously quirky and intensive. He starts off with a rough idea of the plot and then gathers together the cast. He works very intensively with small groups of cast members, and together they construct the characters. They spend long periods discussing in great detail tiny aspects of that character's life, their likes and dislikes, and what has got them to the present moment. At the end of this, each actor knows their character intimately, and can react to situations confidently in character as they start to improvise scenes.
One very important point that he made is that each actor is only allowed to know what their character knows, so there is a great deal of secrecy in rehearsal. This means that -- when they are improvising -- they react naturally because they genuinely don't know what will happen next. Leigh related an example during the rehearsals for the film Vera Drake, where the family were sitting around the table having a meal, and the police knocked on the door. The actor playing Vera's husband had no idea at that point that Vera was an abortionist, so when the police officer read out the charges, he was genuinely shocked. Similarly, they build up the characters chronologically, so if a piece involves a brother and sister, and the sister then marries and later has an affair with another man, the brother and sister characters will be fully constructed before the husband and lover.
After all this improvisation, during which the dialogue and script emerges from the collaboration between Leigh and the actors, the actual filming takes place. But it all remains very natural and believable, because it has been built from the characters' reactions to events. One of the fascinating things that Mike Leigh said in the interview was that he can't actually write anything without seeing it. He sees the screenwriting process as completely collaborative, and a work in progress, much like a novel which is drafted and re-drafted repeatedly as it is written.
I'm not an actor (and don't want to be), but I think that it must be really rewarding (if a bit terrifying) to act in a Mike Leigh film and play such an active part in the creation of the film.