What's left for South Park is to concentrate on things it didn't have when it was riding high in the late '90s, like strong characters. Originally the characters were differentiated only by basic traits (Cartman the fat kid, Kyle the Jewish kid, Kenny the kid who dies). But since then, the characters have developed in unexpected ways. An early running gag about Cartman's anti-Semitic slurs against Kyle has turned into a strangely complex relationship, with many episodes — including the three-part "Imaginationland" episode that formed the centerpiece of South Park's most recent season — built around Cartman's obsessive attempts to score a victory over his rival. It's a love-hate relationship that recalls Archie Bunker and Mike Stivic on All in the Family, one of Parker and Stone's favourite shows.

The show has also grown by giving more storylines to characters who were underused in the late-'90s glory years. Several episodes last season focused on Randy Marsh, Stan's macho but neurotic father, or the kids' teacher, Mr. Garrison, who underwent a sex-change operation. But the most important bit of character development is the rise of the boys' classmate Butters, who didn't even speak in the early seasons. Butters, described by Stone (who provides his gee-whiz voice) as embodying "permanent innocence," is out of whack with the hip humour that made South Park a trendsetter; he'd be like a '50s sitcom child if it weren't for the fact that his innocence always gets him hurt and abused. But as one of the most well-defined and likeable characters on the show, he's become useful for stories that would be out of character for the other boys, particularly in episodes where Cartman tries to cheat or exploit him. "You've got to come up with some new characters, with some new ways of looking at the world," says Stone, "or you'll just die."

South Park has also improved visually, even though the characters are still mostly shown from only one angle. Parker and the animators have livened up the bargain-basement animation with shadows, special effects, and elaborate subjects like a Buck Rogers parody or a mock disaster movie about the lives of head lice. Stone says they're doing better than they did on bigger budgets: "If you look at the South Park movie which we did in '99, and then at the Imaginationland thing we did last season, directorially and cinematically it's worlds beyond what we were doing, even in a movie format. Scenes that would have been simple proscenium staging now are these cinematic things. And that's because Trey as a director has grown so much."

All this means that no matter how ridiculous the stories get — and they still portray a world where Bono is secretly a talking piece of feces — the core of South Park is strangely conventional. Stone could be describing Friends when he says that a good South Park episode is one that "centres on a really solid story involving one of the characters. Those are the ones that appeal most to us, because we're storytellers at heart." Unable to cause the controversy it once did, South Park has filled a void left by the decline of the sitcom: stories that play off our familiarity with the characters. An episode like last season's "Le Petit Tourette" was funny not because it found comedy in Tourette's syndrome (the issue was dealt with somewhat respectfully) but because it was so in character for Cartman to fake having the disease so he'd be free to say whatever he wanted. South Park by now is mostly about how our favourite characters react in certain situations. It just happens that these situations include facing a mob of homeless zombies or getting illegal immigrants to do their homework for them.

At this point it doesn't seem likely that South Park will ever get back to the cultural position it occupied in the late '90s. For one thing, it doesn't have much of a legacy: whereas The Simpsons spawned many imitators and Family Guy has been successfully copied a number of times (including American Dad, from the same producers), South Park remains a stand-alone hit. Other cheap cartoons with a similar sensibility, including some on Comedy Central (like Li'l Bush), have never been particularly popular. And Parker and Stone are no closer to coming up with a second hit; instead they've announced plans to produce other people's shows, including a U.S. version of Canada's Kenny vs. Spenny. But Stone seems to think that he and Parker benefit from the fact that they don't have to be cultural icons: "We've done it, we've achieved it, so now it's like the pressure's off." If the new season is up to South Park's recent level, it may prove what no one predicted back in 1997: South Park does better work when it's not expected to be cutting edge.