In an unusual dramatic turn, Sarah Silverman stars as Laney, a troubled, drug-addicted mother, in ”I Smile Back.” (Jeong Park/Broad Green Pictures)

On the surface, Sarah Silverman’s role in “I Smile Back” couldn’t be more different than the comedy that made her famous. She is best known for getting laughs by saying the unsayable — poking fun at old people, using racial slurs, joking about rape — and delivering her lines with a disarmingly sweet voice and a beatific expression.

“I Smile Back,” on the other hand, is entirely free of laughs. It is a bleak drama, and Silverman is astonishing as Laney, a mother of two who is married to a sweet husband and who suffers from depression that sends her spiraling into drug addiction, alcohol abuse and extramarital affairs.

But there is a common thread between the Silverman we have known for a decade and the one whose gut-punch of a portrayal lands on the big screen this weekend: Her performances make people really, really uncomfortable.

What is with Silverman and making audiences squirm?

“My dad was one of those dads that taught his daughter swears, and so when I was 3 I would scream out swears in the middle of the market,” she explained during a recent phone conversation. “I got this reaction of total approval from grown-ups despite themselves, and it was so addictive — it gave me so much glee.”

That explains Silverman’s brand of comedy. But “I Smile Back” isn’t about the validation of landing a punchline. It is about something more profound, and Silverman admits that she is fascinated with the forbidden for other reasons, too.

“There is something about exposing taboos that makes them not taboos anymore,” she said. “Darkness cannot exist in the light. When you put light on things, it changes what they are.”

On the surface, the movie, based on a novel by Amy Koppelman, looks like another case of suburban ennui, between Laney’s big SUV and bigger house, the school lunches and the family dinners. But her motives are more nuanced. The taboos in this case surround the difficulties of parenting, especially for a woman with severe anxiety and abandonment issues. Laney hugs her children so hard they complain they can’t breathe, and she only half-jokingly tells her daughter “never leave me.” Her fear that something might happen to her kids is crippling. She lives in a land of what-ifs: What if she passes down her depression genes? What if she screws up her children? What if she can no longer see them? Laney’s nightmare of an imagined future becomes a self-
fulfilling prophecy, and the only way to cope is to numb herself using whatever substances she can get.

Silverman’s performance is painfully raw. At one point, Laney stands naked in front of a mirror, sizing up her flaws. Later, in a scene that’s nearly impossible to watch, she uses her daughter’s stuffed bunny to do something unprintable. Reading the script and realizing that what was ahead of her was scary, Silverman admitted, but she made the decision to not be like Laney. She wasn’t going to stress about the future or the possibility of failing. She just took the job moment by moment.

Anyone who read Silverman’s memoir, “The Bedwetter: Stories of Courage, Redemption and Pee,” knows that the comedian has a history of depression, and that helped inform her role. But it wasn’t easy. Silverman said her emotions are “very tightly packed inside” her and letting them out meant that she had no urge to goof off between scenes. She just sat around holding her feelings on her lap. Exhausted after a day of filming, she would have only enough energy to go home, wash her face and crash, ideally in front of a “Law & Order” marathon.

“You know, you always hear that Tom Hanks can be the life of the party, joking around and then they call ‘action,’ and he’s Captain Phillips, and he’s mind-blowing and brilliant,” she said. “I’m not able to do that yet.”

But after this role, she might get more chances to try. She had wanted to do drama for a while, but it was hard for people — even her agents — to see her as anything other than a comedian.

“I think they kind of didn’t see it, and they were like, ‘Well, we would need to have tape on you,’ and it’s like: ‘Well, how can I have tape of me doing drama if I can’t get a chance?’ ” she recalled.

She even tried to get her agents to use a clip of her from “The Aristocrats” as an example of dramatic acting. It is an infamous scene in a documentary about the world’s dirtiest joke. Among the many comedians who put their singular spin on the gag, Silverman’s made the biggest waves because her joke culminated with a deadpan admission that radio and TV host Joe Franklin raped her. (He didn’t.)

“In my mind, and I still believe this, that was a viable drama tape for someone who had no tape on their drama work because, even though you laugh when you watch it, who I’m being doesn’t know she’s in a comedy, and it’s very serious to her,” Silverman said.

But Koppelman saw something in Silverman few others could (as did filmmaker Sarah Polley, who cast Silverman in a supporting role as an alcoholic in the 2011 movie “Take This Waltz”). When Koppelman heard the actress talk about her depression during an interview with Howard Stern, the writer instinctively knew she had found her lead. The actress seems a little incredulous the way things worked out. She didn’t have a plan, but somehow everything fell into place so that she could stretch herself creatively.

“It’s these two women in my life — Sarah and Amy — who were able to imagine me as something they hadn’t seen me do before,” she said. “I’m so grateful for that.”