What would a middle-aged person do upon learning she has a year, maybe 18 months to live? The liberating, philosophical and, yes, comic aspects of such a diagnosis coalesce in “The Big C,” a Showtime series adding Laura Linney to the pay channel’s gallery of quirky leading ladies in filmed half-hours that are more dramatic than funny. Linney invests her character with the requisite anger and confusion, but she’s the lone lifeline of humanity amid a talented cast mostly saddled with caricatured roles. Interesting in its conceit and watchable for what Linney brings to it, the show works too hard at whimsy.

We join the story very much in progress: Linney’s Cathy Jamison — a married high school teacher in Minnesota — has already been informed she has Stage IV melanoma and has chosen to forgo aggressive treatment that, at best, would buy her only a little more time. Instead, she sets out to make the most of what life she has left.

As part of that decision, Cathy initially opts not to disclose her condition to anyone — including her husband (Oliver Platt), who she has tossed from the house; and son (Gabriel Basso), both understandably confused by her behavior. This leads to slightly awkward exchanges with her doctor (Reid Scott), the one person to whom she can confide.

In a way, Cathy’s determination to accomplish — well, precisely what exactly isn’t clear — in the time available begins to feel slightly selfish, inasmuch as her decision to shield loved ones is inevitably temporary.

The supporting cast includes John Benjamin Hickey as her brother Sean — a street-corner agitator who rails against global warming and insists on eating garbage to protest all the food Americans waste — and “Precious” star Gabourey Sidibe as an overweight student Cathy becomes determined to help.

If only — as created by Darlene Hunt, with Jenny Bicks functioning as showrunner — these peripheral players weren’t such one-dimensional constructs. Sidibe is particularly ill-served by strained, sitcom-y dialogue, at one point accusing Cathy of “‘Blind Side’ fantasies, where the uptight white bitch tries to save the black kid.”

Platt, Hickey and Phyllis Somerville (as Cathy’s cranky neighbor with an adorable basset hound, the perfect breed to suit the show’s ambling, bittersweet tone) are good enough to prevent their characters from toppling over the edge.

Still, “The Big C” too frequently falls into what has become a kind of Showtime trap that might be called “Weeds” syndrome, where beyond the attention-getting premise (pot-peddling mom, multiple-personality mom, pill-popping nurse mom, now cancer mom), there’s a surplus of quirkiness for its own sake.

To be fair, these series might benefit from weekly viewing as opposed to concentrated gulps (the first three episodes were made available), allowing an audience to spend time with the intriguing protagonists, then vacate the premises.

In the final analysis, “The Big C” gets an “E” for admirable effort but still feels like a squandered opportunity. Given the chance to explore what truly matters in life, the show ultimately provides little more than a showcase for a terrific actress, while treating death like the next slightly zany frontier.

The Big C

Showtime, Mon. Aug. 16, 10:30 p.m.

  • Production: Filmed in Los Angeles (pilot) and Connecticut by Perkins Street Prods., Farm Kid and Original Film in association with Sony Pictures Television. Executive producers, Darlene Hunt, Jenny Bicks, Neal H. Moritz, Bill Condon, Vivian Cannon, Laura Linney; co-executive producers, Mark Kunerth, Michael Engler, Merrill H. Karpf; producer, Lou Fusaro; director, Condon; writer, Hunt.
  • Crew: Camera, Tobias Schliessler; production designer, Richard Sherman; editor, Brian A. Kates; music, Marcelo Zarvos; casting, Julie Tucker, Ross Meyerson. 30 MIN.
  • Cast: Cathy Jamison - Laura Linney Paul - Oliver Platt Sean - John Benjamin Hickey Marlene - Phyllis Somerville Adam - Gabriel Basso With: Gabourey Sidibe, Idris Elba, Reid Scott.