On the second page of his script for "Vanya and Sonia and Masha and Spike," Christopher Durang describes Sonia as Vanya's "step sister, early 50s, living with him in Bucks County. Discontent, upset, regretful." What he gracefully does not spell out, at least not in the shorthand description, is what comic qualities the character contains. Partly this may be that, in the course of the evening, Sonia attends a party and discovers her dormant gift to charm others: her dark markings are innate, but the light ones need activation. And partly this may be that Durang knows that when Sonia is played by Kristine Nielsen, as she is in the current LCT production, the comedy will emerge regardless.

Watching the play again the other night from the audience, I experienced a renewal of appreciation for Nielsen's gifts, along with a related understanding of why Durang has written so many roles for her. His reasons may be myriad, but here's the one that I keep coming back to: she can convey a thing and its opposite at exactly the same moment. In her hands, there need be no demarcation between sad and happy, low and high, distraught and delighted. Onstage, she is a walking embodiment of Sondheim's lyric from "Company": You're sorry-grateful/Regretful-happy/Why look for answers/Where none occur?" Nielsen conveys that kind of constant ambivalence, and reminds us that most of our waking moments are not single unalloyed feelings but mixtures of them: while we're content to have the delicious first sip of the morning's coffee, we're also worried about our dental appointment in the afternoon. 

One of Nielsen's chief methods to express the coexistence of conflicting emotions is surprise. In Sonia's monologue in act two of "Vanya," for example -- a scene of which I hesitate to reveal too much -- Nielsen keeps subverting the expected response. Sonia has just been asked to dinner by a man, over the phone, and she says: "Saturday? Well I'm not sure, let me check my book." Were this a standard TV sitcom, with simple-sitcom acting, the character would put the phone down and feign a five-count while looking thrilled to have an offer. Not Nielsen. She holds the phone away, and gives a demonstration of those warring emotions I just mentioned. In a flash, she provides the whole life history of the character -- a woman dutifully tucked away in the boonies while longing to have a taste of life's banquet.

To be fair, this moment is not Nielsen's alone. Durang indicates Sonia's indecisiveness in the script: "She feels nervous about saying yes, wonders what to say." A playwright cannot always count on having a Nielsen in every production, and so furnishes an instruction or two. But even when the Sonia is another high-caliber actress, I'd be amazed if she brought to the part what the current interpreter does. Nielsen is a genius at making her character funny while rooting her in reality. 

Nielsen makes playing the effortful Sonia appear effortless. But make no mistake: the skill required is tremendous. Such is the actress's talent that her Maggie Smith imitation would slay the audience even if we didn't have Maggie on our minds right now from "Downton Abbey." And so, as the run moves into its final performances I find myself experiencing a Sonia-like ambivalence: I am sad to see the engagement end, but happy to salute a master and his muse: Durang and Nielsen. 

Brendan Lemon is the American theater critic for the Financial Times and the editor of lemonwade.com.