In relation to the plays of Tennessee Williams, "plastic theatre" refers to the use of props, staging, and lighting to highlight certain events or characters as well as convey abstract ideas.
We see this theatrical technique used to good effect in Cat on a Hot Tin Roof. The principles of plastic theatre can be observed in the setting of the Pollitt plantation, which positively reeks of decadence and decay, with the seemingly illogical arrangement of furniture representing the inexorable decline of a once-great family.
Big Daddy may like to think that he's created a haven of stability and traditional values in his home, but one brief glance at the set tells us that this is not the case and that he's deluding himself. This radical disjuncture between the conventions of plastic theatre and the family values that Big Daddy preaches serves to highlight his hypocrisy and his unwillingness to face the truth of what's happening to his family.
As well as telling us an awful lot about the state of the Pollitt family, plastic theatre techniques also give us a privileged insight into the psychological states of the damaged characters on stage. The lighting, set design, and props combine to represent, to a considerable extent, the never-ending torment that eats away at each of their minds.