I have more or less followed the work of Mike Leigh since seeing Secrets and Lies, one of my top ten favorite movies ever. I knew vaguely about his directing method of rehearsals and improvisation and not using a formal script, but this documentary filled in more details about his methods and a lot mroe besides.
Another of his hallmarks is reusing performers in multiple productions, so some of the commentary is given by actors and actresses I have seen many times, both in and out of his films.
If you know Mike Leigh’s work you should like this video biography. If you aren’t very familiar with his efforts, you might start there.