At a time when filmmaking grammar has become increasingly defined by manipulative shot-by-shot storytelling, writer-director Christopher Guest has for the past seven years been quietly advancing the radical notion that audiences are smart enough to find the joke on their own. Starting with "Waiting for Guffman," then in "Best in Show," and now with "A Mighty Wind," Guest has perfected the art of the improvisational ensemble comedy. He lets them rip and riff for hours, finally editing their performances into eccentric, caustic and humane studies of self-delusion, thwarted dreams and -- always -- hope.

"Waiting for Guffman" was about a troupe of small-town residents staging a musical in honor of their community's sesquicentennial. "Best of Show" followed nine dog owners as they competed in a national championship. "A Mighty Wind" chronicles the reunion concert of three almost-great folk music groups as they pay tribute to their recently deceased manager. Like its two predecessors, "A Mighty Wind" takes the form of a documentary, with all the medium's awkward interviews, candid moments and revealing encounters. And like the earlier films, "A Mighty Wind" features Guest's cockamamie cavalcade of players, including Eugene Levy, Catherine O'Hara, Michael McKean, Parker Posey, Fred Willard, Bob Balaban and the director himself. But where "Waiting for Guffman" and "Best in Show" featured strivers burdened -- or blessed -- by illusions of greatness just around the corner, "A Mighty Wind" is about musicians who actually attained a certain degree of fame.

"The illusion here is that we're folk performers," says Harry Shearer, regarding the Folksmen, the acoustic trio that he, Guest and McKean first portrayed on "Saturday Night Live" 20 years ago. In the movie, the group's only hit is a song called "Old Joe's Place," which reached No. 17 on the charts before the group disappeared. "Really we're pop performers in folk dress, in folk guise. But we weren't going down to Alabama recording field hollers."

"And we weren't political," adds McKean, who with Shearer, Guest and Levy sat down in a Washington hotel suite last week to publicize the film. "It's a vestige of a generation before, where we heard about those stories but we weren't on the front lines. We weren't blacklisted."

"Except by the public," says Shearer.

Indeed, the folkies of "A Mighty Wind" aren't so much musicians as marketing phenomena, packaged by their manager -- the late Irving Steinbloom -- to cash in on what in the 1960s looked to be the next big thing. With their wholesome, gee-whiz personas, the Folksmen could be the Kingston Trio's fatuous cousins; the New Main Street Singers -- a nine-member group (or "neuftet") that features Posey, Jane Lynch and John Michael Higgins -- suggest the devil spawn of the New Christy Minstrels and Up With People.

The most popular of Steinbloom's acts was Mitch & Mickey (Levy and O'Hara), whose love affair and subsequent breakup recalls the real-life 1970s British duo Richard and Linda Thompson. In "A Mighty Wind," Mitch shows up at the reunion a damaged soul after years in a mental institution. His frizzled hair, halting voice and wild, frightened eyes (crossed, as always, for maximum comic effect) suggest any number of burned-out '60s refugees cadging quarters and cigarettes with open guitar cases at their feet. Meanwhile, Mickey has married a catheter executive named Leonard Crabbe and lives a quiet life in the suburbs.

Mitch and Mickey's first hug might qualify as the most awkward hello in cinematic history, and their subsequent story line isn't played for a single laugh. Mitch is the most vulnerable and poignant character yet in a Guest movie, and by the time the two sing their big hit -- which, in their heyday, culminated with a chaste, romantic kiss -- the emotional stakes are unexpectedly high.

Levy, who wrote "A Mighty Wind" with Guest, acknowledges that the serious plot strand was something of a departure. "It's a really fragile way to go, and we really couldn't goof around," he says. "I remember Catherine's reaction when she first got the outline and she was going, 'Umm, I don't see too many laughs here.' "

Rather than bring the audience down, Mitch and Mickey's love story only deepens and warms "A Mighty Wind," which is populated with Guest's usual panoply of dreamers, schemers and losers. Shearer's character sports one of those Dr. Seuss chin-only beards and white socks with Birkenstocks.

McKean's character -- the most ambitious of the trio -- hasn't left the 1970s, psychically or sartorially. And Guest actually shaved his head and grew a mustache to portray Alan Barrows, the most ambivalent Folksman.

As with all his other movies, the humor in "A Mighty Wind" is sometimes obvious -- one of the leaders of the New Main Street Singers is a former porn star named Laurie Bohner. But the characters are too subtly and compassionately drawn to qualify as caricatures. Guest and Levy -- who also collaborated on "Waiting for Guffman" and "Best in Show" -- spend months writing each person's back story, carefully constructing all of the lives and the three-act structure through which they intersect. Cast members are given the 20- to 30-page scene-by-scene outline, and the first time they see each other it's in front of the camera -- no rehearsals -- with all the dialogue improvised.

Guest scouts every location but invites cast members to compose their characters' looks with makeup and wardrobe, and to bring in any props that will help them nail down their characters. (Jim Piddock, who plays Mickey's husband, brought in real-life family photographs to scatter around the Crabbe house.) On "A Mighty Wind," for 24 days each cast member created his or her own world, one that, Guest recalls, "all the other actors really enjoy watching and being around, as opposed to the insulation of a [regular] film set, where you're in your trailer and they call you and you come out. This is a good hang."

"When we were shooting the concert it was a great hang," McKean breaks in, "because we'd go in and watch for a while and then somebody would say, 'Hey, Balaban and Mike [Hitchcock] are going at it, you should go watch that. . . ."

"Yeah, people are coming out like it's the opening night of a Broadway show saying, 'Oh my God, you've got to go down and see what they're doing,' talking about other people in the cast," says Shearer.

"And meanwhile in the parking lot, in downtown Los Angeles in the middle of the night, we're out there with guitars."

"Playin' and singin' . . ."

"It was the best."

Guest, McKean and Shearer have been playing music together for decades, both as the legendary heavy-metal band Spinal Tap in Rob Reiner's "This Is Spinal Tap" in 1984, then as the Folksmen on "Saturday Night Live" the same year. (They were last seen opening for Spinal Tap -- in other words, for themselves -- in 2001.) But not everyone in "A Mighty Wind" was as seasoned a performer. Posey learned the ukulele for her part, and O'Hara learned the autoharp. Because, he admits, "I can't lip-synch," Guest decided to film the climactic concert live, over two days at the Orpheum Theatre in Los Angeles; nerves were understandably taut when the time came to film the scene.

"We had an audition session," Levy recalls, turning to Guest. "At one point you came in and said, 'I want to hear your stuff, I want to hear what you and Catherine are doing, see if you can play and sing together.' "

"I didn't do that as an audition, I saw it as a possible firing opportunity," Guest interjects with a perfectly straight face.

"It was nerve-wracking, it really was," Levy says. "The music is so critical in this movie. These groups had to be credible. You actually had to have a sound that the audience would buy as credible, that these people could have been folk stars. So we always had those concert dates circled on the calendar as kind of D-Day for us."

More like V-Day: The concert is a sweet, corny, awful triumph, emceed by Irving Steinbloom's nervous son, John (Balaban), and overseen by a Yiddish-quipping public television executive named Lars Olfen (Ed Begley Jr.) and the New Main Street Singers' manager, Mike LaFontaine, played by Willard with his usual brand of spiky smarm.

In fact some of the greatest joy in the film is to be found in the smaller roles, like the theater manager portrayed by Michael Hitchcock, or a publicist with brainpower as indeterminate as her accent, played by the scene-stealing Jennifer Coolidge. Their moments are what make "A Mighty Wind," like all of Guest's other movies, not just great fun but -- as time will most likely tell -- enduring as well. They reward serial viewings, because he packs so much thought and detail into every scene, because he trusts his actors to take those details and make something of them, and because he trusts the audience to be as observant as he is. Guest is brilliant enough to be capable of great cruelty -- he can wield a blade with lethal accuracy -- but he has too much soul simply to revel in his characters' suffering.

He might be our only legitimate heir to Preston Sturges and Billy Wilder; through both the way he makes movies and the flawed, funny people he makes them about, Guest has proven to be this era's master of humanist satire.

A Mighty Wind (87 minutes, at area theaters) is rated PG-13 for sex-related humor.

Eugene Levy and Catherine O'Hara add a serious note to Christopher Guest's satirical pseudo-documentary.Director Christopher Guest, left, Michael McKean, Eugene Levy and Harry Shearer discuss "A Mighty Wind," a documentary-style movie about a reunion concert of three high-profile folk music groups.