“We realize he’s crying” must be among the scariest stage directions an actor could find at the top of a script. How do you get from zero to tears with no context?

That’s the challenge Donald Margulies puts before the actor playing George in “Lunar Eclipse,” his new two-character play at Shakespeare & Company in Lenox, Mass. And wait, it gets harder. As we will soon learn, George, a Midwestern farmer in late middle age, is stony and unsentimental, the opposite of a weeper. Yet as the lights slowly rise on his dark night of the soul, there he is anyway: a heaving, racking torrent of sobs.

Is it thoughtless to say how lucky we are that the heaving and racking come from Reed Birney?

It’s certainly lucky for Margulies and his somewhat overripe tear-jerker, which opened on Sunday. Often threatening to drown in a generalized wetness, the play benefits immensely from the detail and discipline of Birney’s superb performance. He doesn’t so much produce emotions as shed them.

After nearly 50 years onstage, that’s no surprise; he’s won Obie, Drama Desk and Tony awards for his no-nonsense, full-sized approach, in which acting is the side effect of his insight and inhabitation of character. What’s surprising, and a bit scary, is that he has played such a variety of men so vividly: a vile journalist, a penitent philanderer, a conniving cross-dresser. How many feelings does he have inside him?

And how many ways of turning them off? As soon as George’s wife, Em, arrives, the tears and all other signs of vulnerability get ruthlessly shut down. George is so ornery and curt that even after 50 years or so of marriage, Em (Karen Allen) must dance around him in search of some opening to his secret grief. Emotional intelligence has turned her into a spelunker.

If you do not know such pairs from real life, you probably know them from the theater. “On Golden Pond” and “The Gin Game” both offer variations on the “crusty old man bickering with woman who knows better” template. Also like “Lunar Eclipse,” those plays try to corral their rambling contents within the bounds of a thematic fence: the months of a summer, the deals of a deck.

Even more heavy-handedly, “Lunar Eclipse” uses the phases of planetary alignment as both plot and poetics. Its seven scenes (followed by a coda) are called “stages” and are described in pedantic voice-overs: “Stage 1. Moon enters penumbra. Penumbral shadow appears.”

The framing adds nothing, in fact detracting from a story that could stand to be tighter and better grounded in reality. Margulies, so expert with urbane, artistic and moneyed characters — he won a Pulitzer Prize for “Dinner With Friends” and was a finalist for “Sight Unseen” and “Collected Stories” — is not as convincing with farm folk. (“Wild Turkey does a mighty fine job keeping you warm,” George says as if he were on “Hee Haw.”) I had to nod in agreement when I read a program note admitting that “astronomical liberties have been taken for dramatic purposes.”

Astronomical in both senses. Yet despite the liberties, “Lunar Eclipse” remains affecting when its staging, by James Warwick, gets granular. In the middle of the night on which it takes place, George has come to a “sacred” spot on his farm (the needlessly rotating set is by John Musall) to watch the earth’s shadow eat up the moon. Em has followed him there with “provisions”: blankets he does not want and hot chocolate he reluctantly accepts only when she pours it into his tin cup of bourbon. She carefully applies bug spray to the neatly turned cuffs of her jeans (costumes by Christina Beam); he swats insects from his face throughout (crickets by the sound designer Nathan Leigh).

These concrete details operate in helpful contrast to the back story revealed at regular intervals as the eclipse progresses over the next 90 minutes. We learn of beloved dogs buried nearby. Of a troubled sore-spot of a son. And of a new fear: that the early signs of dementia are beginning to cloud George’s mind.

That the night is likewise too cloudy for a perfect viewing seems apt. (The very dim lighting is by James McNamara.) We see pretty well into George by observing his resistances, but Em, despite Allen’s astute performance, is underwritten ane mostly reactive. Her one expressed grief, about a tragedy now years in the past, cannot stand up to George’s million pesky, present annoyances.

Perhaps that’s Margulies’s point — and a way of making meaning of the eclipse, which otherwise seems like a McGuffin. “This may come as a shock to you,” George says with his usual asperity. “Even though we’ve been married forever, we’re two separate people.”

In other words: We cannot know each other. Marriage, even if not loveless, is not equal. As with planets and moons, one spouse generally revolves around the other, doing more work and yet, come crunchtime, left in the obliterating shadow.

Lunar Eclipse
Through Oct. 22 at Shakespeare & Company, Lenox, Mass.; shakespeare.org. Running time: 1 hour 30 minutes.