Best answer to that question ever. In this performance of "Astronomy Domine," accompanied by an amusing interview, you get to see the first inkling of how they would go from this to giant arena rock with huge floating pigs and circular movie screens with computerized light shows. Ballrooms: the gateway drug for arena rock.
And I love how Roger states he doesn't feel aggressive towards his audiences. This is the man who would go on to first visualize The Wall as an audience being bombed to bits and applauding the whole while.