GARRISON KEILLOR IS probably the most polarizing national radio personality this side of Rush Limbaugh. Either like millions of devoted NPR listeners you have an abiding affection for the Lord of Lake Wobegone. Or, like me, you relate more to Homer Simpson, who in one memorable episode of “The Simpsons,” tried watching a TV show modeled after “A Prairie Home Companion” only to jump to his feet and pound his TV in frustration, shouting “Be more funny!”

Those who enjoy Keillor’s laid-back shtick can be expected to turn out in droves on June 9, when Picturehouse releases Robert Altman’s celeb-studded adaptation of the NPR show. The film is opening wide — at least by Altman standards — in 650 theaters.

It’s an ambitious release plan, particularly on a weekend that pits Guy Noir and his folksy circle of singing Minnesotans against “Cars” and “The Omen,” two studio juggernauts that will plow into multiplexes behind tens of millions of dollars’ worth of TV advertising. Picturehouse, as a general rule, doesn’t buy that kind of TV inventory for its movies. But the niche label is working hard to generate a commodity more precious than advertising: word of mouth.

As part of its grassroots marketing of “Prairie,” Picturehouse plans to screen the film for employees of 100 NPR stations nationwide in the days leading up to its release. Picturehouse chief Bob Berney likens the word-of-mouth screenings to those that were staged for Greek organizations and for church groups before the release campaigns he oversaw for “My Big Fat Greek Wedding” and “The Passion of the Christ.” The idea, he told me, is “to get the early advocates and opinion-leaders to see the movie, and to make these early adopters feel like they’re part of the team that made the movie.”

“Companion” may not inspire the same devotion as a film about the resurrection of Christ. But if there’s anyone who lends himself to a word-of-mouth marketing campaign, it’s Keillor, an alarmingly prolific novelist and journalist who wrote the screenplay for, and appears as himself in, the film. Keillor is a one-man word-of-mouth machine. And “Companion,” like “The Passion” and “Greek Wedding,” has a target audience that doesn’t conform to the usual media clichés: an older, affluent cross-section of the population that tunes into NPR religiously even as national radio ratings continue to slip and fragment.

As radio and TV ads, and other traditional marketing tools, grow less efficient, word-of-mouth has become an object of intense fascination to marketers. But it’s hard to define, particularly in Hollywood, where marketing campaigns are all about buildup and anticipation. Fox didn’t bother to screen “X-Men: The Last Stand” for thousands of Marvel Comics fans before last Friday. When you can afford to send cast members Hugh Jackman, Kelsey Grammer and Halle Berry to New York aboard an 844-foot amphibious assault vessel and get product placement on the final episodes of “American Idol,” who needs word-of-mouth screenings?

PR doyenne Peggy Siegel has been holding word-of-mouth screenings for the last 25 years, but these events are usually reserved for films like “Good Night and Good Luck” and “United 93” that profit from the early endorsement of news organizations and tastemakers. Last week, Siegel staged a word-of-mouth screening and celebrity auction for “The Devil Wears Prada,” part of a series of highly targeted screenings that Fox is planning for the film in advance of its June 30 release. Improbably, last week’s screening was attended by Vogue editor Anna Wintour, the woman on whom Lauren Weisberger’s novel is allegedly based, as well as by Donna Karan, Candice Bergen and various Park Avenue socialites and gossip mavens. “You try to find a core audience to validate the film,” Siegel told me. “Anna Wintour validated this film by showing up.”

DESPITE THE APPARENT effectiveness of such ploys, word-of-mouth marketing has long been overlooked by major consumer brands largely because it didn’t seem to lend itself to the sort of metrics (gross ratings points, costs per thousand, etc.) that a TV campaign does. The Word of Mouth Marketing Association has been working hard to change that perception. Next month, it’s holding a conference on “Word of Mouth Basic Training,” otherwise known as WOMBAT, to study the ways in which this marketing practice can, as WOMMA head Andy Sernovitz puts it, serve as a “plannable, trackable part of the marketing mix.”

WOMMA has its own peculiar vernacular. Its members use terms like WOM Units to describe the spread of social influence from opinion leaders to their friends and neighbors and colleagues. But for all this goofy adspeak, WOMMA’s marketing theories hinge on an intriguing concept: that networks of human beings can be massaged and manipulated the same way a mass media campaign can be. “The future is going to be what I call people networks,” said Dave Balter, founder of a Boston-based marketing agency BzzAgent which employs some 180,000 volunteer word-of-mouth agents from coast to coast. “We’re turning our system of agents into a media form. Companies can access them as they access other media forms.”