To help explain how “The Country House” could be so mediocre despite an award-winning playwright’s classic inspiration, a first-rate production and an exemplary cast, let’s start with a scene involving all six characters in the play. The great actress Anna Patterson (portrayed by the great actress Blythe Danner) has gathered her extended family in Anna’s country house in the Berkshires near the Williamstown Theater Festival for the first time since Anna’s adult daughter Kathy died of lung cancer a year earlier. Kathy’s widower Walter, a director of action flicks, has brought along his new, much younger girlfriend Nell, an actress. The others ask Walter and Nell how they met:

Nell: We met at Starbucks.
Susie [Kathy’s daughter]: Ew, you’re kidding
Nell: I know
Elliot [Kathy’s brother, a failed actor]: Didn’t see that coming.
Walter: I’d just finished auditions for the picture I’m about to do…
Elliot: You mean people actually…