The equation between theatre and a character may be the occupational hazard of an actress,but it is an understandable one when the gallery of characters from Shakespeare to Williams, excitingly realized by Miss Leigh on stage and screen, is recalled. This Miss Leigh also exhibited a gracious eagerness to bestow credit wherever she believed it reasonably belonged. Since the starring presence of Sir John Gielgud did not save last year’s Tiny Alice, some critics suspect that it is her presence drawing crowds to Ivanov, a play as stuffy as the former was confounding.