Bertolt Brecht believed that theatre should not merely entertain its audience but make them think politically. To achieve this effect, thought the German playwright and poet, a play should not be polished – but jarring. Actors should break out of character to address their audience, plotlines should be broken up and interrupted. In one memorable phrase, he described his ideal play as one that could be “cut into individual pieces, which still remain fully capable of life”. A new exhibition at Raven Row in London shows how literal the author of The Threepenny Opera was being when he came up...