Two for the show

“DON’T [screw] this up for me, Laura.”

Harsh words for a sister costar about to share the spotlight in a joint interview. Overhearing them, an outsider might well wonder: Is Sarah Silverman being a jerk, or just joking?

But that’s precisely what her fans like about her particular kind of humor. You just don’t know. Like Sacha Baron Cohen (“Borat”) she presents an obliviously offensive persona that serves, we hope, to comment on cultural taboos. The difference is that Baron Cohen performs in full character while Sarah basically plays herself -- using her name, her voice and, in her new TV series, “The Sarah Silverman Program,” her sister.

In the Comedy Central show, which debuted Thursday, Sarah is a childish, self-centered, foul-mouthed moocher named Sarah; Laura, though four years older in real life, plays the younger, more responsible sister, a nurse named Laura, who helps rescue her from inevitable jams.

Not to be confused with a standard network sitcom, the series is a loose fantasy, accented by singing, animation and plenty of Sarah’s gross-out humor.

The scripted series, written by a team of six, is “mostly made up,” she said. “Except that we’re really sisters and Duck is my dog and those guys [Steve Agee and Brian Posehn] are all my friends.” Agee and Posehn play nerdy gay neighbors Steve and Brian. Jay Johnston is Laura’s boyfriend, Officer Jay. (Duck was renamed Doug, Sarah said, to protect his anonymity.)

Compared with Baron Cohen, Sarah said, performing as herself has been somewhat limiting in previous stand-up routines (including her film, “Jesus Is Magic”), guest appearances (“Saturday Night Live”) and nasty girlfriend parts in films (“School of Rock”). Because this Sarah is a more “layered” character, it affords an opportunity to push boundaries even further.

As retiring as Sarah is explosive, Laura has played more soft-spoken parts in TV shows, including “The Comeback” and “Dr. Katz, Professional Therapist.” The sisters appeared together in “Jesus Is Magic” (2005) as friends/colleagues/rivals trying to make it in Hollywood.

In person as on-screen, they appear to like each other, laughing in their deadpan way over each other’s jokes, bragging about each other (Laura is a whiz at science and languages, Sarah said; Sarah, Laura said, is a “savant”) and praising each other’s looks (Sarah resembles a young Katharine Hepburn; Laura is the Cate Blanchett type).

As it turned out, the order not to ruin things is a running in-joke between the siblings. After Sarah issued her command not to mess up the interview, Laura promised as usual, “I won’t.”

Growing up, both wanted to be performers, but as the extrovert, Sarah stole the spotlight. “She wanted the same things as me,” Sarah said, “but I had sort of already called it.”

“It took me a long time to come out as an actor,” Laura said. “I thought they’d all go, ‘That’s horrible, she has no talent. She’s too shy.’ It’s a different experience for me than for Sarah; we come at it from different angles.”

Sarah said Laura is great at comedy because “she plays it very honestly and real.”

Basically, she said, “Being a comedian is like being gay: You’re just born that way, and it’s beyond your control.”

Unconventional upbringing

RAISED by a book-smart mother and street-smart dad, the girls grew up in Bedford, N.H., with an older sister, Susan, now a rabbi, and a stepsister, Jodyne, a writer in Hollywood. “Sarah was the baby and always cute and always funny and we had fun with her,” Laura said. “Then my parents got divorced.

“It was a crazy situation,” she said. “Sarah lived with my mom. My older sister lived with my dad because she wanted to. And they got joint custody of just me. So I went back and forth.”

As in the show, Laura played a caretaker role for Sarah, she said. “I even did her laundry. I didn’t want the kids at school to make fun of her if her clothes weren’t clean.” She also made Sarah call up the operator and sing to her. It was a way to prepare her to perform in public, she said.

Their father, Donald, remarried and established a highly structured home while their mother went back to school and underwent a personal “renaissance” of freedom, friends and love affairs, the sisters said. “She grew out her leg hair,” Laura said. “She started wearing two different-colored socks,” Sarah said.

“I did,” said Beth Ann O’Hara, who also remarried. Sarah had a lot of chutzpah as a child, she recalled. “At 3, she’d be in a room with adults and charm them. ‘Want me to sing a song?’ ” Her father also coached her into saying jokes with swear words mostly to annoy his mother, who turned out to laugh after the initial shock, she said.

“[Sarah’s grandmother] would call up and say, ‘Sarah, I made brownies for you.’ Donald would coach her to say, ‘Shove them up your [behind], Grandma.’ She’d laugh and laugh and Sarah learned you could shock people and no one would hate you for it.”

In addition to learning dirty-word jokes from her father, Sarah also watched R-rated movies at a young age. “We were not sheltered. From pop culture, we were sexualized at an early age,” Sarah said, quickly adding that it wasn’t in a literal way.

In the show, the sisters’ parents are dead, not because it’s true but rather to avoid ... complications. Their father, however, did appear as an extra in one episode, and if the series is renewed, her mother will be invited too, Sarah said.

Not surprisingly, some find Sarah’s aggressive humor off-putting; they should prepare for some of the strongest material on TV. Sarah said stand-up comics such as she often need increasingly strong material to maintain the excitement about their work. “A Catholic schoolgirl, someone may brush against her shoulder and she’d get excited. A porn actress might need three midgets and a goat to feel anything....”

Sarah said Comedy Central has not diluted her edgiest material in “The Sarah Silverman Program,” although “there were many times Standards and Practices had problems. We were able to win battles with them because we could cite ‘South Park,’ ” she said. “They set so many precedents with what you can get away with on a basic cable channel.”

At the same time, there are signs she’s learning when to back off.

Such as when she’s onstage and gets the uneasy feeling that she might be bullying an audience with her aggression. At the same time, though, “I don’t want to preach to the choir too much. It’s not edgy or risky if you’re not risking anything. It’s not like I’m trying to be Andy Kaufman and get everyone against me.... It’s better to be the ignoramus, the stupid one, to be the underdog.”

Because she’s spending more time with the children of her boyfriend, talk show host Jimmy Kimmel, she said she has started to watch her language around them. “I know not everyone raises their kids the same way. I have to kind of relearn things.”

And she’s also had to learn to take what she’s dished out. In the premiere episode of the new series, Laura falls for Officer Jay but worries that her scofflaw wild card sister will cause trouble. In her best responsible sister voice, it’s Laura who tells Sarah: “Don’t screw this up for me.”

lynn.smith@latimes.com