On South Park, nothing is off limits. Foul language, gleefully rendered scatological humor, biting social commentary, Kanye West coming out as a gay fish—it’s all pretty much par for the course. Over the past 19 years, Comedy Central’s animated juggernaut has proven to be the most enduring, consistently funny comedy on air. Its closest contemporary, The Simpsons, continues to experiment—but most critics would agree that show has already seen its Golden Age. South Park, however, continually manages to innovate in ways that really pay off—like last season’s robust, successful foray into serialization.

When Matt Stone and Trey Parker first started work on South Park, they never could have known how big it would become. In its first season, the series became Comedy Central’s best-rated show; by its second, it was more-watched than any non-sports show in basic cable history. Ratings have fluctuated since then, but South Park still draws both audiences and critical acclaim. Its most recent Outstanding Animated Program Emmy came just three years ago, for an episode called “Raising the Bar”—which weaves together plotlines including Cartman embracing his obesity, Token making a secret reality series about his friend called Here Comes Fatty Doo Doo, and James Cameron embarking on a deep-sea expedition, for some reason.

Perhaps South Park’s most refreshing element is its refusal to take itself too seriously—no matter how seriously its fans take it. The series has effectively lampooned everything from Scientology to rain-forest conservation to P.C. culture, but often manages to do so without any real agenda. The adults are crazy; the kids are profane and clueless; the situations they find themselves in are so ridiculous that one has to imagine their creators laughing at anyone looking too hard for a deeper meaning. Perhaps the show’s long-running textual introduction says it best: “All characters and events in this show—even those based on real people—are entirely fictional. All celebrity voices are impersonated.....poorly. The following program contains coarse language and due to its content it should not be viewed by anyone.”

With Season 20 arriving September 14, Vanity Fair chatted with Parker and Stone to find out how they’ve managed to keep South Park fresh for almost two decades—as well as why last season’s Donald Trump episode made them nervous, how comedy and P.C. culture have changed since their show began, and what’s coming next for their quiet mountain town.

It’s impressive enough that South Park is in its 20th season—but this has also been one of the most consistently satisfying comedies on TV. How do you guys keep the pedal on the gas every season?

Trey Parker: For whatever it’s worth, we still do everything we did when we first started. We really haven’t handed the show off to anyone. So for better or worse, it’s not like people trying to constantly recreate what South Park was—we’re constantly trying to figure out something new for it to be. And I don’t even know if it’s totally conscious—it’s just that we come in, and we don’t want to do the same thing, because it sounds boring to us. So we come to every season trying to say, “All right, what can we do totally differently?” And that’s part of what makes it fun for us.

Matt Stone: And I think we’ve been doing it long enough, hopefully, that we’ve gotten a little bit better at what we do—like anybody who’s done something long enough. So we need to keep it exciting for ourselves, and maybe we’ve gotten a little bit better.