The watcher

All families, happy or unhappy, big or small, monogamous or polygamous, are alike in this way: Somewhere in the carton, there is a bad egg. And often, there are a few, slightly cracked and making life sticky for everyone else. But on TV dramas, these sour, malignant, or merely irritating relatives are good news indeed, at least for viewers who aren't looking for a nice hour of "7th Heaven." They put the dis, the funk, and sometimes even the shun in TV family dysfunction.

This summer's wiliest and most wonderful domestic weasel is Chloe Sevigny , who's back for a second season as sister-wife Nicki on "Big Love." Sevigny's performance is a show-making turn, as she brings underhanded menace to the HBO drama and gives each episode its richest psychological layering. I'm not sure "Big Love," about Bill Henrickson of Salt Lake City and his three wives, would hold the same intrigue without Sevigny's Nicki at its center, masterfully inflaming others with her deviousness and her dolor. Sevigny makes Nicki into a quietly compulsive instigator, an ace triangulator who can set her fellow sister-wives against each other -- and toward her -- with minimal effort.

Sevigny (it's pronounced "Seven-knee") rarely lets herself smile freely in this series. Indeed, in her movies, most of them indies, Sevigny generally wears a wary exterior. When she began her acting career in the controversial 1995 skater-culture movie "Kids," as a sad, uncommunicative HIV-positive girl, she immediately established herself as a performer whose poker face can tell a story. She keeps her cheeks in check, her mouth slightly pursed, like a cat sitting on a counter, observing, waiting. And yet she manages unexpected range within this tentativity.

In "Big Love," she lets only a tinge of insolence into her eyes, and puts a slight flare into her nostrils, to make Nicki's malice crystal clear. You can feel the accumulating power of her rage long before it emerges. Nicki looks almost tame, but scratch the surface and she's a mess of animal need -- Janice Soprano camouflaged as a dutiful wife.

In 1998's "The Last Days of Disco," Sevigny operated with the same near-expressionlessness but projected a radically different character -- an insecure, sincere single in New York. And in 1999's "Boys Don't Cry," she was the heart of the film exactly because she held back, very incrementally letting her Lana feel love for a woman transitioning to a man. Her slow turning toward Hilary Swank's Brandon Teena mirrored ours. Sevigny was nominated for an Oscar for "Boys Don't Cry," and she should have won, although she did get an Independent Spirit Award and prizes from a number of critics' groups.

Oh, sure, the other two sister-wives of "Big Love" are endearing enough. Jeanne Tripplehorn is one of the show's anchors as top wife Barb, a noble cancer survivor who isn't fully convinced about the whole polygamy thing, which requires her to be submissive to Bill. And Ginnifer Goodwin is sweet as Margie, the naive child-wife, whose ability to keep the family's polygamy a secret is as feeble as Nicki's is fierce.

But Sevigny stands out as the interestingly twisted one who's hard to like, who challenges viewers to find sympathy for her despite the conniving and the chafing. She's the juicy one. With Nicki, there's always an ulterior something or other going on. You have to keep watching her, just as you'd keep an eye on a snake in the grass. She couldn't be appealingly rational like Barb or perky like Margie; she's just too dark and alien.

In Sevigny's hands, Nicki is a woman who is smart and hungry for control, but strictly raised to be subservient. She has spent her life suppressing her needs, to abide by the sexist code of the fundamentalist Mormon compound where she was raised. So her appetites fester in secrecy, and then she tries to satisfy them on the sly -- thus her raging shopping addiction, which she tried to hide last season until debt outed her to her spouses. It's a perfect role for Sevigny, since Nicki is forever using her face as a mask to the truth.

The actress also brings out Nicki's classic Freudian bind. As the daughter of "prophet" Roman Grant, she clings to him and his morality, and she lectures her fellow sister-wives about staying true to "the principle" of polygamy. She has no faith in love as the glue in a family. "How will we survive the bad times on just 'love'?" she asked Barb recently, like a child. But then Nicki is drawn to Bill's gentler, more loving approach and the responsibilities of adulthood. Now that Roman and Bill are enemies, thanks to business tensions and compound power struggles, she is more torn apart than ever.

Sevigny also makes Nicki funny, lest I make her sound one-dimensional. She can be like a superstitious old biddy, a young version of Bill's hot-headed mother, Lois, played with comic spite by Grace Zabriskie. And Nicki's bullying of Margie -- "I'm second, you're third, get it?" -- is right out of the Little Rascals. After Barb, whom she calls "boss lady," briefly moved out of the house earlier this season, Nicki couldn't wait to scoot into Barb's seat at the head of the table in a jealous pang -- think Jan in "The Brady Bunch" moaning "Marcia, Marcia, Marcia." Recently, in a power play, Nicki forgot to deposit enough money for Barb's grad-school tuition check. Oops.

Nicki's clothes are an ongoing visual joke, too. Sevigny is known as a fashionista, and early in her career she was portrayed in the New Yorker magazine as an "It Girl" (by writer Jay McInerney). She still appears at fashion shows and on red carpets, and in interviews she remains well-versed in designers. But on "Big Love," Sevigny is forever buttoned-up to her chin in "Little House on the Prairie"-style dresses that speak of Nicki's early compound life. Her hair is braided or pony-tailed, her features de-glamorized.

There's not a hint of the It Girl about Nicki, unless we're talking It circa 1880.

Matthew Gilbert can be reached at gilbert@globe.com. For more on TV, visit boston.com/ae/tv/blog/.