"It's all about the Goldsworthys."

Tilda Swinton's laugh is only slightly rueful as she winds up a photo session in front of "Roof," sculptor Andy Goldsworthy's installation of stacked-slate domes at the National Gallery of Art. While Swinton, who like Goldsworthy lives in Scotland, poses in front of the contemplative mounds, her feet occasionally grazing their surface, it's clear that the museum's curators and security personnel are far more excited about the well-being of the art than the mere presence of a celebrity.

Of course, it doesn't help that Swinton, 44, exudes not one whiff of actressy self-regard. In town to promote her new movie, "Thumbsucker," which opens Friday, the muse of artists and fashion designers alike keeps a low profile in the hushed museum, stopping only once to sign an autograph. She describes her fan base as a "secret handshake society," and it's true that, since making a stunning debut in Derek Jarman's "Caravaggio" in 1986, then delivering similarly potent performances in Sally Potter's adaptation of Virginia Woolf's "Orlando" and the post-feminist parable "Female Perversions," she has garnered a following composed of the discriminating and passionate few, filmgoers whose admiration of Swinton borders on worship of her patrician, androgynous beauty and keen intelligence.

But that has changed in recent years, as Swinton has begun to appear in bigger, or as she calls them, "industrial" films. Once a sort of mascot of independent film at its most intellectual and adventurous, Swinton has become a memorable presence in movies as diverse as the journalistic thriller "Adaptation," the sci-fi drama "Vanilla Sky" and the comic book fantasy "Constantine."

Swinton's co-stars in those films were Nicolas Cage, Tom Cruise and Keanu Reeves, respectively, and the fact that hers is increasingly mentioned in the same breath as such marquee names comes as something of a surprise to her fans.

"For me, industrial film is really interesting spy work, and then I go home and do what I really do," Swinton says of her brushes with Hollywood. She is having lunch in one of the museum's cafes, sipping cranberry juice and stabbing at a small seaweed salad with a pair of chopsticks, her cool blue eyes fixing an interlocutor with a level gaze. Her hair, dyed white for her role as Jadis the White Witch in the upcoming "Chronicles of Narnia," is uncoiffed and showing dark roots; her nails aren't "done"; and she's dressed with the sort of polyglot insouciance that only a naturally beautiful, 5-foot-11 child of the aristocracy can pull off: a long-sleeve, blue-and-white silk sheath from Lebanon, a belt that Swinton fashioned this morning from one of the leather straps of the Vivian Westwood tartan handbag she carries and a worn pair of strappy, sexy, powder-blue Manolo Blahniks.

"The creativity is months ago with big-budget filmmaking," Swinton continues, speaking of her spy work in Hollywood, "which doesn't mean they're not creative, it just means that the pacing is different. It also means that you . . . don't have to make friends with chaos in the way that you have to in low-budget filmmaking. In fact, very often chaos has to be your best friend, because there's no way of avoiding it."

Swinton befriended chaos yet again while filming "Thumbsucker," a low-key comedy in which she plays the mother of a 17-year-old high school senior (played by newcomer Lou Pucci) who still sucks his thumb. The film, an adaptation of a novel by Walter Kirn, is the first feature of music-video director Mike Mills, who sought out Swinton for the role of a middle-aged woman seeking to sublimate her ambivalence about motherhood and aging by indulging in a fantasy romance with a famous soap-opera actor (Benjamin Bratt). "Thumbsucker" co-stars Vincent D'Onofrio, Vince Vaughn and Keanu Reeves.

Swinton was one of the first people to sign on to the project, and her support was so steadfast throughout the film's preproduction ups and downs that Mills made her an executive producer. "What brought Mike and me together is that we were both interested in a kind of inarticulacy, which is quite unfashionable in drama," Swinton says. "The idea that we don't necessarily have the words to say what we want to say. Particularly in a family situation, where very often we never get to say what we really want to say, and very often words are the last place where we are articulate. . . . We wanted to look at that tone, that unfinished-ness and mess and [create] a sort of frayed experience rather than a sort of completed one."

Having observed Swinton in her role as producer, actor and now adjunct publicist for "Thumbsucker," Mills says: "I don't think she identifies herself as an actress so much. I think she's found a way to be sort of an emotional activist. She's always trying to create a world where it's okay not to always know what you're doing, it's okay to be unsure, it's okay to be all different kinds of people. She's making a vote for being permissive, for not being judgmental, and for being more human in the sense of being less perfect."

Indeed, Swinton becomes increasingly animated while discussing those themes as they play out in "Thumbsucker," which as a coming-of-age film mostly subverts the notion of coming of age. "How are we supposed to do that?" Swinton cries. "What age are we supposed to be coming of? Why should a 17-year-old feel the need to come of age, when it's very difficult to come of age when you're 40?"

Swinton became a parent herself seven years ago, when she gave birth to twins: daughter Honor and son Xavier; with the children's father, artist John Byrne, Swinton divides her time between a home in the seaside town of Nairn (pop. 11,000), in the Scottish Highlands, and the family's "wilderness" house farther north. "People constantly ask how [I've] changed as a mother, and I don't feel I have changed," Swinton says. "It's just that another self exists." The challenge, she adds, "is trying to keep one's self alive as well as the self that is nurturing and looking after everybody else."

"Wow," Swinton murmurs thoughtfully. "Wow, wow, wow, wow." She is standing in the National Gallery's mezzanine, gazing down at the velvety black oculi of "Roof," which Goldsworthy and a team of British wallers built along the East Building's north side last winter. Resting snugly between the gallery's floor-to-ceiling windows and an outdoor wall, the nine meticulously layered structures evoke not only the neoclassical architecture of the surrounding city but also Scotland's primitive dwelling cairns. "It's very funny, because it does make me homesick," Swinton says of the installation. "However site-specific it is, it's so Scottish."

And make no mistake: Swinton is Scottish through and through. Hers is one of the oldest and most distinguished families in the country and her father, Sir John Swinton, was a major general in the Scots Guards (Swinton went to boarding school in England -- with the future Princess Diana -- and studied political science at Cambridge). But she's had no trouble conquering the Yank accent to play a completely believable series of American women, starting in 1996 with the aggressive attorney in "Female Perversions," up through her much-lauded performance in 2001 as the desperate mother of a teenage boy in "The Deep End" and, most recently and perhaps most indelibly, her portrayal of Penny, a former lover of a Don Juan played by Bill Murray in Jim Jarmusch's "Broken Flowers."

Swinton's scene, which comes late in the film, is less than a minute long, but it's one of the most effective moments in the movie, a shard of pure fury that pierces through the film's mood of quiet aimlessness. For the performance, Swinton wore a long, dark wig that obscures half her face, somehow making her rage more fierce; even her hair seems angry.

Swinton says that, unlike the other actresses in "Broken Flowers," she didn't discuss the fictional past relationship with Murray before filming the scene. "It was somehow so obvious to all of us, when we shot that scene, what happened," she says. "But none of us ever shared it." As for the hair, she says: "That's sort of how she had to look and that's how she looked in the script. The real question is why Jim asked me to do it, because he could have asked somebody who really looked like that."

Jarmusch himself still isn't quite sure why he gravitated toward Swinton for the role. "Those things are very mysterious," he said in a recent telephone conversation. "I just really love her presence on screen, ... and I didn't want to diverge from this look I had in my head while I was writing it." As for the startling physical change, he said, "I just knew her spirit would come through that. And she liked the idea of being transformed."

If Swinton was the counterintuitive choice to play Penny, her casting as Jadis the White Witch in "Chronicles of Narnia" -- due out this Christmas -- could not be more spot-on.

With her exotic, otherworldly looks and persona of chilly reserve, she's the ideal choice to play a character that Swinton jokingly calls "the ultimate white supremacist." The film, which marks the live-action debut of "Shrek" director Andrew Adamson, will bring to the screen for the first time the cherished children's stories written by C.S. Lewis in the 1950s. Surprisingly, Swinton herself did not grow up with the novels.

"For some reason they passed us by," she says. "But it's great for me, because it meant I didn't have any baggage. I just read it when they asked me to do it. I'm only beginning to realize now what a big deal it is."

Swinton's own children won't be seeing "Chronicles of Narnia." "Why should they?" she says. "It's got their mother playing the epitome of all evil." On the other hand, she adds upon reflection, "It's a great mothering exercise to act out the epitome of all evil, because then I think that I'm actually not doing too badly!"

Back at "Roof," Swinton stands where she is told while a photographer scrambles precariously around the domes' razor-sharp openings. A stylist briefly hovers over the actress, but only to apply a little powder and a swipe from a tube of Neutrogena lip balm. That's it, and Swinton -- a frequent fixture in the pages of fashion's most high-end periodicals -- is ready for her close-up. As the photographer clicks away, the actress is confident, utterly at ease and naturally radiant.

For the moment at least -- and for this cinematic season, certainly -- it's all about Tilda Swinton.

"Thumbsucker" star Tilda Swinton with the sculpture "Roof," by fellow Scotsman Andy Goldsworthy, at the National Gallery.Tilda Swinton obliges autograph seekers earlier this month in Toronto.Swinton, right, with Charlotte Valandrey in 1993's "Orlando."Benjamin Bratt and Swinton in "Thumbsucker," an independent film by Mike Mills that opens Friday.Swinton with "Broken Flowers" co-star Bill Murray at Cannes, where the Jim Jarmusch film won the festival's second-place honor, the Grand Prize.