The ‘not/but’ is a technique Brecht introduced to help actors understand the choices available to their figures. That is, he encouraged them to show not only what their figures were doing, but also what they weren’t doing as well.
In, say, Ibsen’s A Doll’s House, Nora decides to leave her husband and children at the end of the play. In terms of the ‘not/but’, she does not stay with her family, as she would be expected to, but leaves them. In a naturalistic production, the actor may simply leave, but in a Brechtian production, the actor can show the decision she has taken not to stay as well.
The importance of the ‘not/but’ can’t be understated. It opens up the action because it robs it of a sense of inevitability. Nora had the option of staying with her family and in signalling this to spectators, she invites them to ask why it is that she didn’t remain. The moment of decision can provoke curiosity from an audience and suggests that whatever happens in a drama is the result of deliberateness; there is always a choice, but the options are often loaded.
The ‘not/but’ is not only useful for the audience; actors can also probe what their figures don’t do as way of understanding why they do other things. In raising the question of decision and choice, the ‘not/but’ approach can give precision to action on stage. Indeed, Brecht suggested that an actor could base a figure completely on this principle.