The most sizzling actress to come along in ages is sitting across from me in a diner on Manhattan’s far West Side, doing what all the best new thespians do: Bewitching her hapless interviewer. She has the kind of face that you can’t stop staring at—beautiful, in an unconventional way. Thoughts and feelings move quickly across it, like clouds.

Even if you hadn’t seen this star on the big screen in Julie and Julia, or on the off-Broadway stage at Café La Mama in La Vie Matérielle last winter, you’d know you were in the presence of a serious talent. “To suddenly find a new, young actress of that caliber….” marvels La Vie’s director, Irina Brook. “It’s like A Star is Born. She’s a really marvelous actress, with incredible charisma.”

“I was this American who felt French and lived English,” she recalls.

But here’s the thing: This particular star— Joan Juliet Buck—is actually in her early 60s. And although she is relatively new to her craft, she has been receiving serious attention since Julie and Julia, in which she played Madame Brassart—“a bitch from hell,” as she describes the flinty director of the legendary cooking school, Le Cordon Bleu. “You have no real talent for cooking,” Buck’s character hisses at Meryl Streep in the film, her accent seethingly Gallic, before marching away in her crisply tailored suit. It’s a restrained performance, one that walks a taut line between drama and comedy. The results are riveting.

Raised in Europe, primarily in London, Buck attended the Lycée Français. “I was this American who felt French and lived English,” she recalls. Her mother, Joyce Gates, was a stage and radio actress, and her father, Jules Buck, a noted producer. After high school she attended a cours prépératoire in Paris—“Marxist training,” she calls it—then zigzagged to Sarah Lawrence. After that, it was New York and the beginning of a writing career. Two novels followed as did marriage and, later, divorce.

She worked for the American, British and French issues of Vogue and likes to say that she has been writing for the magazine since she was a baby. In 1994, she landed as the editor-in-chief of French Vogue, and took the magazine into rarified, if not necessarily fashionable, realms, at one point devoting an entire issue to quantum physics. She speculates without apparent bitterness that that particular issue just might be why she was axed, finally, in 2001. She’s now a contributing editor at American Vogue.

After Paris, Buck gravitated to Santa Fe where she began to study acting in earnest. “I was sick of the black-and-white of writing,” she says. She worked with Ruth Zaporah, the developer of an improvisational training process known as Action Theater; later, in New York, her coaches included Elizabeth Kemp of the famed Actor’s Studio, and others. Today, Buck lives alone, just blocks from the diner, in an apartment that reflects her fascinating, mercurial mind. It is dense with books and intriguing objects, from a half-used packet of Nicorettes (they aren’t working) to an exquisite, antique Chinese screen. C.G. Jung’s coveted, posthumous Red Book is placed dead center on her coffee table.

“Acting is about listening and watching, the magnification of attention,” Buck says, holding up a glass of water and focusing intently upon it. As she says this, I can’t help but notice that here, in this diner, it is she who draws all the attention. She orders espresso, then cantaloupe, and a string of employees stop by to pay homage, including a young cashier who cheerfully agrees to a studying session with her in one of the booths later in the week. (“We read back and forth when I’m studying my lines,” Buck explains.)

It’s easy to see why Nora Ephron, an acquaintance, cast her for Julie and Julia after a screen test, and why Brook decided to use her in La Vie Matérielle—an extraordinary production that the director describes as “women and cooking and Marguerite Duras and Virginia Woolf.”

Buck’s latest project is a new version of Henry James’ The Aspern Papers. Set in today’s Venezuela, this low-budget, independent film puts “Jamesian themes into a new context,” according to its director, Mariana Hellmund. Buck plays Mrs. Prest, a character she describes as “an entrepreneurial socialite.” The fact that Buck is the most unconventional of ingénues is a strength, Hellmund says. “That she’s in her 60s, that she’s real, and that she hasn’t had face work done, has great appeal.”

As we leave the diner, heading into New York’s hot, clamorous streets, Buck stops to light a cigarette, never interrupting her stream-of-consciousness commentary, which ranges from the out-of-work actors in front of a pair of nearby buildings to the “lovely, cheap hairdresser up the road.” She adds, “I know I’m a character actress.”

She’s not, she says, expecting a major career. But I’m not so sure about that. Buck’s fan base is growing. And when director Irina Brook refers to herself as “Joan’s No. 1 fan,” it seems clear that she’s going to have to take her place in line.

Plus: Check out more of the latest entertainment, fashion, and culture coverage on Sexy Beast—photos, videos, features, and Tweets.

Penelope Rowlands has contributed to numerous magazines, including Vogue and Architectural Digest. Her most recent book, A Dash of Daring: Carmel Snow and Her Life in Fashion, Art, and Letters , is a biography of the legendary editor of Harper's Bazaar.