Among the many reasons to see Robert Altman’s new film “A Prairie Home Companion” this June, Meryl Streep’s performance might just top them all: viewers have the once-in-a-lifetime opportunity to actually witness what Streep would be like as a mediocre entertainer. The imperial actress plays against her austere character type, transforming into second-rate country singer Yolanda Johnson on set of the final broadcast of Garrison Keillor’s live radio variety show.

“My character’s supposed to be a kind of a half-assed singer, so the mistakes were ok,” Streep says at a press conference at Brookline’s Coolidge Corner Theatre, hours before receiving the Coolidge award. “That’s how I rationalized it, anyway.”

According to Altman, those off-key imperfections color the film: “I was looking for mistakes to improve the script,” he explains.

The climax of a month-long Streep celebration, Streep joined fellow actors John C. Reilly, Kevin Kline, and director Robert Altman after a preview screening of the film, to discuss both the craft and the award, which honors original and challenging film making.

“This award is really celebrating not so much me as what a small art theater, independent (means), and that creature has to be preserved,” she says, under the restored Art Deco ceiling of the seventy plus year old movie palace. “I think it’s a really important thing, and I’m proud to help out. Some of my really good friends are associated with the theater, and I’m happy to support that.”

Susan Orlean, the New Yorker writer and author of the book “The Orchid Thief”—who Streep played in its 2002 screen meta-adaptation, “Adaptation”—serves on the Advisory Panel of the Coolidge.

In “Prairie,” Streep delivers another genuine, effortless performance, Midwestern accent and all. Coupled with Altman’s directing style—which many recognize by its characteristic natural feel—the film adopts a graceful charm. Altman uses several unique technical and philosophical approaches to create his signature effect.

Multiple cameras—at least three, mostly three, and occasionally four, according to Altman—work each scene, preventing actors from catering to a single one. “It turns it into a theatrical experience,” says Reilly, who also complimented Altman’s use of mirrors to further complicate the shot. “You’re not aware of the frame so much anymore.”

In that setting, Altman believes, actors focus more on their performances. Streep adds, “You’re just living it.”

To bolster the organic vibe, Altman abandons the set script; with his guidance, actors instead improvise as the tape rolls. Streep recalls the first day of filming: “Lily [Tomlin] and I just looked at each other. We couldn’t remember what to say, so we just said whatever came into our minds.”

She remembers that Altman encouraged them. “He kept us going and he was unafraid, and it made us unafraid,” she says.

Altman also maximizes his actors’ talents by shooting essentially boundless scenes, full of uninterrupted stretches of dialogue during which characters speak simultaneously. “There’d be long, meandering takes, and all I could think was, ‘Surely he’ll say ‘Cut!’’” says Streep, laughing to herself. “It’s bravado directing.”

Of course, some themes intentionally make the final cut of “A Prairie Home Companion.” A persistent carpe-diem message pervades the film; characters learn not to focus on the past—“there is no silence in radio,” one points out—and that in the future lingers uncertainly.

At the same time, the audience sympathizes with the outdated variety show, and its closing, the end of an era, implies that something wonderful is really being lost.

“It’s a very nice tension between saying go forward into the future…and also looking back, the tension of what’s lovely and needs to be preserved,” Streep says.

“Prairie” addresses that tension on several levels, notably by casting the budding multi-media sensation Lindsay Lohan as Streep’s angst-ridden daughter. The 19-year-old drama queen earned the respect of the veteran cast, in spite of recent negative press attention for her eating habits and partying.

“I think it’s very hard to be her, offscreen, but I think it’s very wonderful to be her onscreen, because she is remarkably talented and alive on camera,” Streep notes.

“She’s a natural,” Reilly follows.

“And she’s been a natural all her life,” Altman adds.

“She wants it,” Streep says. “She wants to be good.”

To Lohan’s credit, the film’s multi-generational ensemble cast and crew carries quite a lot of clout—20-some Oscar nominations among them. According to Altman, it was easy to manage the concentrated star power. “When there’s so many, they police themselves,” he says.

Altman shot the film in just 23 days, a relatively speedy production time. And while his techniques may be a little unconventional, the end result satisfied Streep, who calls it “just so alive.”

Still, even to the most experienced actor, filming poses new obstacles.

“The most challenging thing was the very first day,” Streep confesses about her first movie with Altman. “When I came to work, the director told me, ‘We’re down here in this basement and there are three cameras and there are 17 mirrors and we’re going to shoot the first 10 pages.’”

She acknowledges her status, as well as her stage fright: “As intrepid as actors are, and as long as a career may be, you still are terrified of walking in front of people.”

But as 13 Oscars nods will attest, for now, Streep can handle the pressure.

—Staff writer Lindsay A. Maizel can be reached at lmaizel@fas.harvard.edu.