The Pulitzer Prize committee may have never erred more egregiously than it did in favoring “Harvey” over Tennessee Williams’s first masterwork, “The Glass Menagerie.” But handled with care, as it has been in this Roundabout Theater Company production, this winsome comedy about a lovable eccentric can cast a satisfying spell. Mr. Ellis’s amiable staging—which features expert supporting performances from Jessica Hecht, as Elwood’s dithery sister, Veta, and Charles Kimbrough, as the eminent psychiatrist she hopes will lock her troublesome brother up for good—strikes the right, gently dizzy tone. Most important, Mr. Parsons carries the weight of a role immortalized on film by the inimitable James Stewart as lightly as Elwood does the hat and coat he keeps on hand for his furry companion. Mr. Parsons possesses in abundance the crucial ability to project an ageless innocence without any visible effort: no small achievement for an actor in these knowing times.