Most of the laugh lines in the new Tina Fey-Amy Poehler comedy Sisters don't seem particularly funny on the page. It isn't inherently hilarious when Poehler, playing a nurse, tells a man with a music-box ballerina wedged up his ass that it's unfortunately going to take a while for the device to wind down, because it's "Swiss-made." Or when Fey leers at a gardening neighbor that she's looking for someone who's capable of working on other people's bushes. Or when John Cena, as a monolithic drug dealer, growls, "My safeword is ‘keep going.'" Not hilarious in print, anyway.

But all these bits work onscreen, because they're all delivered with such straight-faced, innocent conviction, and because they follow a simple dynamic: they're small parts of larger comic set pieces, all purposefully stumbling toward a peak of relentless, hilarious momentum. Potentially short bits sprawl out at length. The characters' banter becomes increasingly nervous and wonky. These scenes needle the audience — "Yes, this ballerina-up-the-ass bit is still going on. He's still deeply uncomfortable, she's still trying to reassure him and failing, and that thing is still spinning in there, playing its tinkly little version of ‘Für Elise.' This might actually go on for the rest of the movie."

It's discomfort comedy drawn out to an excruciating degree until it bypasses awfulness and comes around to funny again — a common enough comedy tactic in the Judd Apatow Age. But Sisters extends the method into the entire arc of the film.

Fey and Poehler star as sisters whose parents (played by James Brolin and the ever-reliable-in-angst Dianne Wiest) have just sold their sprawling Orlando home and moved into a ritzy retirement community. For Maura (Poehler), this amounts to a crisis because she wasn't consulted, and as the kind of caretaker figure who masks deep insecurity and a hefty dose of self-righteousness behind attempts to help other people, she's uncomfortable with the loss of control. For Kate (Fey), it's a crisis because her life is a continual crisis. She's just lost another hairdressing job, and she was hoping to mask her lack of options by moving back in with her folks. Her teenage daughter Haley (Madison Davenport, fresh off a very similar but much more serious role in A Light Beneath Their Feet) is exasperated with Kate's lack of responsibility, adding an extra layer of guilt to her unemployment. For both sisters, it's a crisis because the teenage identities they're still carrying around as baggage are heavily tied up with the house, and they aren't ready to let go just because their parents are.

So after wallowing in the horrifying cutesiness of their perfectly preserved shared childhood bedroom, Maura and Kate plan one last wild rager in the (already sold) house. They're out to relive their teen glories, but they're also saying a defiant goodbye to their past, and flipping the bird at Mom and Dad for having the audacity to live their own lives and make their own decisions. Disaster is inevitable. From the moment they decide to throw the party, the whole movie becomes an exercise in squirm as the situation spirals out of control, with an inevitable crash and burn around every corner.

But there's another dynamic at work, and it's ultimately Sisters' big differentiator: the Fey-Poehler partnership. The SNL and Second City alums' longtime offscreen friendship informs their onscreen chemistry, and boosts their status as awards-presenter favorites and frequent cultural commentators. Fey and Poehler do their mean-girl sniping publicly, so everyone else can feel like part of their bestie relationship, joining in on the cutting whispers aimed at the rest of the world. There's always been something pleasantly and counterintuitively inclusive about their intimacy.

That feeling extends to Sisters, where their characters are frequently frustrated with each other's considerable faults, but still so mutually giving and approving that they encourage each other into entertainingly awful behavior. Having a built-in support system leaves them free to play grotesques without fear of judgment: they can try on terrible dresses, harass that hapless neighbor, or pooch out their stomachs to rub them against each other in a public "tummy kiss" without embarrassment. They have each other there to say, "This is not only okay, it's actually an important part of our unique and special identities. We're having fun, and there's nothing wrong with that." There's a joyousness to their bad-girl behavior. Like the similarly contentious but close relationships between Seth Rogen and his besties in Apatow movies, or between Jonah Hill and Channing Tatum in the Jump Street movies, it may not accurately reflect any real-life friendship that's ever existed. But that's part of what makes it absorbing: the fantasy of having an endlessly approving partner in crime, and all the freedom and self-confidence that comes with a trustworthy confidant.

In that sense, Sisters feels like a close descendant of Romy And Michele's High School Reunion, where Lisa Kudrow and Mira Sorvino play a similarly bonded-at-the-hip pair of ditzes facing a similar party where identity crisis underlines every other crisis. But while Maura and Kate share Romy and Michele's loving relationship, they're older and more unsettled. They're clutching at the more confident teenage girls they used to be, instead of some imaginary vision of adulthood. They're also crasser, louder, and above all more sexual. (Among other things, Kate is convinced Maura needs to take advantage of her last chance to get laid in her childhood home, in her childhood bed, possibly with that gardening neighbor, played affably and with virtually no impact by The Mindy Project's Ike Barinholtz.) Sisters never reaches Bridesmaids levels of raunch, and it's admirably removed from that film's fascination with bodily fluids and feces. But it sometimes feels like a film-length version of Bridesmaids' winningly frank, subversive café conversation about oral sex. The humor circles obsessively from boobs to groin and back again, pausing only for cartoon penises drawn frequently enough to rival Superbad.

Sisters packs in familiar comedy faces, especially women: Maya Rudolph gets the juiciest supporting part as a realtor desperate to crash Kate and Maura's party, but Saturday Night Live's Bobby Moynihan is a close second as the party guest who can't stop cracking painfully awful jokes as he struggles for even the tiniest bit of validation from a crew that was already tired of him back in high school. John Leguizamo, Rachel Dratch, Samantha Bee, Kate McKinnon, Chris Parnell, and many others pop up at the party, or in other settings. But Sisters never feels overcrowded, because it's so unquestionably Poehler and Fey's show. Director Jason Moore (Pitch Perfect) is as unobtrusive and pleasantly generic as a comedy director can be, and the rest of the cast feels like Styrofoam packing peanuts, cushioning the stars without overasserting themselves. They give the central sisterhood room to breathe, and to dig themselves deeper and deeper into situations that just get funnier as they get more unlikely and more inescapable.

There's a heavy streak of melancholy running just beneath the surface of Sisters, as Kate and Maura ignore their problems, booze them away, or try to rewind to a simpler era. The film gives them enough room to develop past the rough outlines of "control freak" and "loser." But like any Apatow movie, Sisters isn't really about the lip service it pays to life lessons about growing up. It's about people with positive, crackling comic chemistry riffing off each other, and enjoying each other's company in a target-rich environment. It's about inviting audiences to join in the fantasy of the kind of unequivocal love and approval that makes those life lessons look easier, and that melancholy seem navigable. Not every joke works, on paper or on screen. But Fey and Poehler at least look like they're having fun, and they make it easy to get pulled along for the ride, no matter how awkward it gets.

Sisters opens in theaters this Friday, December 18th.