PARK CITY, UTAH: Of all the tidbits shared by Clea DuVall — the beloved indie actor turned writer/director — at the Sundance Film Festival premiere of her debut feature The Intervention, the least surprising was how many of the people she cast as her friends in the movie are her real friends, in real life. “Melanie [Lynskey] and Natasha [Lyonne] are my oldest friends, and Melanie was very much my muse for this character,” she said, “and I really gave the character over to her, because it was very inspired by her and anything she could do with it was so much better than anything I could ever imagine.” That spirit of generosity, of creating roles for her actor friends and giving them the opportunity to really show what they could do, is all over The Intervention; it’s a movie full of duets and group scenes that play like improvisations or acting exercises (in the best possibly way, please don’t let those phrases scare you off). As a result, it contains some of the most full-bodied work to date from some of your favorite character actors.

It’s a story of four couples spending a weekend at a vacation home, and yes, the Big Chill echoes are intended. The focus of the weekend is a longtime-married-with-children couple (Cobie Smulders and Vincent Piazza) who are one of those pairs — you probably know one too — whose relationship has become defined by their constant fighting and insults. Annie (Lynskey) believes its is the others’ duty, as friends, to sit these two down for a “marriage intervention” — to make it clear that they’re miserable, that they’re making everyone else miserable, and that the only sensible thing to do is to end the marriage. But over the course of the weekend, it becomes clear that everyone has hidden vices and little secrets, and that none are exactly involved in models of healthy relationships.

Clea DuVall, Melanie Lynskey, Ben Schwartz, Jason Ritter, and Natasha Lyonne at the Sundance Film Festival premiere of “The Intervention.”Photo Credit: Jason Bailey / Flavorwire

Comedy like this is all about characters, so the opening scenes are a bit on the bumpy side — understandably, as we’re getting to know these people and how they interact. But once these people are established, it’s just pinball; DuVall proves an adroit writer/director, creating durable group dynamics, situating and triangulating these characters off each other. It’s a very funny movie — one of the funniest I’ve seen at this Sundance — without stooping to cheap or obvious laughs. The wit is sly, the laughs rooted in character and behavior rather than easy punch lines. It’s seriously funny; there’s real stuff under these comic constructions, real pain and heartbreak and fear, and DuVall pulls in and out of those serious turns with ease.

And that width allows her actors to really show their stuff. Smulders was something of a breakout star of Sundance 2015, thanks to her nuanced turns in Results and Unexpected; here, she takes what could’ve been a one-note character and finds the desperation and hopelessness that propels her. Piazza doesn’t shy away from his character’s ugliness, but manages to neatly reveal what made him attractive in the first place. Ben Schwartz plays the pain of his character without overplaying it, and easily doubles the intensity of his laugh lines through the split-second precision of his comic timing. Jason Ritter has sort of made a specialty of playing charismatic nice guys, and fills that role here nicely. Alia Shawkat (also very good in the very different Green Room), is the outsider, breezing into the house like a lit firecracker, and it’s fun to watch her go off. DuVall is something of the anchor for the picture, onscreen and off, and grounds it well. Lyonne is sort of underused, but that’s pretty much always the way it goes.

Clea DuVall introduces “The Intervention” at the Sundance Film Festival.Photo credit: Jason Bailey / Flavorwire

The MVP, however, is Lynskey, who’s been doing great work in indie and mainstream movies for two decades now. But she’s rarely had the chance to be as full-on funny as she is here, whether scolding her friend’s too-young, too-sexy girlfriend that “no one loves a Jolene” or looking over the wreckage of their efforts and shrugging, “This is kinda the worst-case scenario right now, huh?”

In the post-screening Q&A, DuVall confirmed that she had indeed been involved in a real-life intervention, with similar results: “It was unsuccessful.” But the film was less inspired by that experience than a product of “really loving the process of filmmaking, and working with actors, and really just wanting to have a bigger piece of the puzzle.” And she didn’t do so on a lark. “I’ve been an actor for 20 years, and I wanted to be a writer before I even wanted to be an actor,” she explained. “I started writing about ten years ago, and then wrote this script, and this was the first script that I really wanted to try and get produced. But I wanted to someone else to direct it, because I was nervous about acting and directing at the same time.”

She needn’t have worried. Sure, there are some first-timer fumbles; an overly tidy ending, for one, and a soundtrack that does too much of the heavy lifting (often rather clumsily). But she’s got real wit and a good eye, and most of all, the kind of affection for actors that allows them to do their best work. DuVall’s got enough credits to have continued in her character actor/scene-stealer vein for years, popping in to films and television shows like a gunslinger, doing her work and then moving on quickly. Instead, she’s begun what may prove to be a very interesting second act.

The Intervention is playing this week at the Sundance Film Festival.