"For “City of God” Fernando looked at 2,000 kids saying their name and some information about themselves on videotape. He then selected 250 and broke them into 6 groups. Rehearsals would run from 8am to 9pm, giving him 2 hours with each group. Initially, he worked on making them less self conscious in front of the camera and building their confidence. While the entire film was scripted, the actors were never given anything to read. Instead, Fernando would give them a scene and have them go work on it as a group. Once they prepared, he would meet with them and shape it, slim it down and feed them lines from the script to throw in. During the shoot, he will often say “keep going” instead of “cut” because actors will often think they are improvising, but resort to the scripted lines making them sound more natural than the initial performance."
A lot more at The Workbook Project.