Over the course of a summer afternoon in 1983, Dahl shifts from one persona — a cantankerous but lovable author — to another self, one kept deeper down. As Dahl whines entertainingly, or pluckily rubbishes a threatening crank caller, Lithgow’s face gleams like an apple; he seems young, even hobbling across the stage with his cane. But, particularly interlaced with his acidic outbursts, Dahl’s use of baby talk grows increasingly galling. By the time the 66-year-old author is crooning, “More more. Yum yum. Plum plum,” to his cook, Hallie (Stella Everett), over his sorbet, the cutesiness has stopped being sweet-old-man-ish. It’s chilling.