“You’re going to be real popular this summer,” Cathy Jamison’s brother Sean tells her in the second episode of Showtime’s The Big C.” (The first debuts Monday at 10:30.)  

“I have no idea who I’m going to be this summer,” Cathy tells him.

Though Cathy’s mood and persona change frequently over the three episodes of “The Big C” I’ve watched, I have a pretty good idea who and what she’s going to be. As played by the always-fabulous Laura Linney, she’s another in a long line of movie and TV characters (recent TV ancestors include Mary-Louise Parker on “Weeds” and Bryan Cranston on “Breaking Bad”) who respond to a life-altering - or, in this case, life-threatening - piece of news by deciding to alter that life even further.

Cathy is a wife (to Oliver Platt’s Paul), a mom (to Gabriel Basso’s Adam) and a high school teacher in suburban Minneapolis. Her life is bland and joyless. In the series’ opening scene, she lets a contractor talk her out of getting the pool she wants so he can expand her deck instead, and it’s clear she’s spent years settling for the wants of others.

But Cathy has a secret: she has a serious case of skin cancer. She tells her rookie oncologist, Dr. Todd (Reid Scott from “My Boys”) that she doesn’t want chemo because she’d rather not lose her hair, but ultimately both doctor and patient acknowledge that the cancer isn’t treatable. (Scott and Linney have a relaxed, completely candid rapport that’s the one part of the show that feels wholly original.) So rather than spend the rest of her abbreviated lifespan being taken care of by others, Cathy decides she’s going to reinvent herself as a free spirit who does what she wants - and who won’t tell Paul, Adam, Sean (John Benjamin Hickey), or anyone else about her condition.

Her rationale - that if her loved ones found out, her relationships with them would change too much for her liking - makes sense for about a half-second, until it becomes clear that radically altering her behavior without telling anyone why is changing things just as much as, if not more than, the truth would. She wants to grow closer to her son, for instance, but her context-free clinginess only drives him away.

But even if it’s a flimsy rationale, it still gives her license to act cuh-razy! She intentionally spills wine on her couch! She does carthweels in the hallway at school! She shoots at a school bus with a paintball rifle! She’s not yet as wacky as Sean - a save-the-planet activist who’s homeless, doesn’t bathe and only eats food that’s been thrown out or is about to be, and whose every scene carries with it the air of self-satisfied whimsy that unfortunately infects most cable comedies - but by the end of the summer, she may be close.

And if the actress playing Cathy wasn’t as talented and committed as Laura Linney, “The Big C” might be unbearable.

Fortunately, the creative team - including creator Darlene Hunt and producer Jenny Bicks, a “Sex and the City” alum who is herself a cancer survivor (she bought a Porsche after being diagnosed) - has Linney, and Platt (playing the same kind of overgrown baby he did on Showtime’s “Huff,” but playing him with relish), and Gabourey Sidibe. And that cast makes “The Big C” seem less trite than almost comforting in its familiarity.

In particular, the show comes most to life whenever Linney is placed with Sidibe, the Oscar-nominated “Precious” star who here plays one of Cathy’s students, a snarky, overweight girl whom Cathy decides to make into a personal project.

When Andrea arrives late to a summer school class and loudly mocks Cathy’s teaching style, Cathy pulls her aside and cuts right through her by saying, “You can’t be fat and mean, Andrea. Fat people are jolly for a reason. Fat repels people, but joy attracts them. You can either be fat and jolly or a skinny bitch. It’s up to you.”

It’s a wonderful moment: an ice queen trying to warm up and reach out, but a bit too impatient and indelicate to do it any other way, and Linney’s joy at being able to say and do whatever comes into her head is infectious. And once Cathy takes Andrea under her wing and offers her a hundred bucks for every pound she loses, Sidibe is just as charming as a young woman slowly realizing that the boundaries of her life are being hastily redrawn.

Like “Nurse Jackie” and “Hung,” “The Big C” isn’t so much a comedy as a half-hour drama with occasional laughs (most of them coming from Sidibe and Platt), and the show is less profound and novel than it seems to think it is. But the performances are strong enough that I want to stick around for Cathy Jamison’s final journey, even if the path feels particularly well-trod.

Alan Sepinwall may be reached at sepinwall@hitfix.com