The entertainment media (of which I am an unapologetic member) loves its trend stories, and one of the popular trend pieces a few years back was about how Showtime had passed HBO as the standard-bearer for quality programming on pay cable. HBO was in its fallow post-"Sopranos" period, while Showtime kept rolling out one attention-getting new series after another, from "Weeds" and "Dexter" through to "Nurse Jackie" and "United States of Tara," which both begin their third seasons tonight at, respectively, 10 and 10:30.   

Somewhere along the way, though, HBO mounted a furious comeback, thanks to the popular success of "True Blood" and critical acclaim for the likes of "Boardwalk Empire" and "Treme." And at the same time, the formula with which most of the Showtime series had been created - some combination of noted character actors plus high-concept premises, often placed in a half-hour format that somehow classified laugh-light series as comedies - has become to seem as conventional and tired as the stuff the broadcast networks churn out season after season. Most Showtime series burn hot, burn bright, then hang around for a very long time, flickering on and off, because it's bad business sense to let them simply burn out. (Case in point: "Dexter," which will never change a darned thing about its formula so long as it's the channel's biggest hit.)

That's not always the case, though, as demonstrated by the contrast between tonight's two returning shows. "Nurse Jackie" exemplifies the stuck-in-neutral, idea-over-execution problems that ultimately plague most Showtime series, while "United States of Tara" has found a way to transcend arguably the channel's most gimmicky premise.

I don't have much new to say about the third season of "Jackie," because the show's strengths and weaknesses are the same as they've always been. Edie Falco is delivering a finely-tuned lead performance as the painkiller-addicted title character, and supporting players Merritt Wever and Peter Facinelli are funny enough that the series actually feels like a comedy from time to time. But other than improving the characterization of Anna Deavere Smith's hospital administrator between seasons one and two, it's a show that has stubbornly refused to - or been unable to - evolve. Jackie is still Jackie, still getting away with the same antics she always did. The first season ended on a cliffhanger where it appeared Jackie was on the verge of being found out, only for season two to largely ignore it. Season two ended with Jackie's husband and best friend confronting her even more blatantly about her drug problem, and... well, let's just say this is a show that doesn't believe much in the idea of consequences.

"Tara," on the other hand, has been very open to the notion of consequences, and evolution. Its conception seemed to represent the Showtime formula to a T: recognizable star (Toni Collette), unconventional premise (Kansas housewife with multiple personalities), shiny creative pedigree (Steven Spielberg came up with the idea, and Oscar-winning "Juno" writer Diablo Cody created the series and its characters), etc.

It would have been very easy for the show to rest on its obvious hook, simply giving Emmy winner Collette a new alter ego to play each year and telling the same kinds of stories - with the same kind of Cody-style quips - over and over again. But while the writers have, in fact, revealed a new personality or two for Tara with each season, the series as a whole has gotten deeper, darker and just plain better as it's gone along.

In some ways, it's not even trying to be a comedy anymore, but that's largely to its benefit. The new season - run by longtime "Tara" writers Brett Baer and Dave Finkel - takes Tara's condition and its origins very seriously. The episodes delve into questions the show either hasn't asked before or has only touched on briefly. Tara's husband Max (John Corbett), sister Charmaine (Rosemarie DeWitt) and kids Kate (Brie Larson) and Marshall (Keir Gilchrist) are all allowed to acknowledge the tremendous emotional burden of loving a woman who at any given moment could become one of a half-dozen other people, and it leads to some tremendous moments for the entire supporting cast. (Corbett's carved out a nice career based on good looks and tremendous affability, but he's really challenged this season and he rises to it.)

At the same time, Tara - with some help from her new psychology professor, played with usual roguish charm by Eddie Izzard - struggles to maintain prominence in and control of a body that's overcrowded with so many colorful characters like '50s housewife Alice and macho Vietnam vet Buck.

"I'm just sick of disappearing," Tara laments, "and being the least interesting person inside me."

The new season starts a bit slow - and gets particularly uncomfortable during a poorly-timed storyline in which Kate's planned trip to Japan is ruined by an earthquake. (As she pouts, Max wryly notes, "200 people died. This may not be about Kate.") But it's all laying the groundwork for a gripping, almost nightmarish second half of the season in which the life of every member of the Gregson family seems in danger of splintering as badly as Tara's psyche.

It's not what the series was at the beginning, but that's what makes it so good. There's plenty of room on television for shows that are going to be the same from week to week and season to season. CBS makes a lot of money on those, and now CBS-affiliated Showtime does as well. But the best shows on television are usually the ones that challenge our expectations of the kinds of stories they'll tell and the sorts of places to which they'll take their characters.

In many ways, these two series are very much like their heroines. Jackie is a stubborn woman who sees no need to alter the course of her very problematic life, and the show around her is just as afraid of change. Tara is someone who's constantly examining and challenging herself, trying to figure out what makes her tick and how to be better, and the show is going on the journey of self-discovery with her.

Alan Sepinwall may be reached at sepinwall@hitfix.com