No one likes the idea of growing older, and anyone who claims he does is lying, either to you or to herself. The anxiety of aging actors is particularly acute, not necessarily because they feel the passage of time intensely but because, having the privilege of watching their faces change over the years, we do. Juliette Binoche, as fictional movie diva Maria Enders in Olivier Assayas' Clouds of Sils Maria, doesn't look old at all, whatever “old” means; if anything, she looks no-age, in that vibrant, alive-to-the-world way. But to watch Maria reckoning with the reality that the actresses yapping at her heels are getting younger and younger is to be reminded that, eventually, we'll all be replaced. That's just the way life works, and it's the way of movies, too. If Juliette Binoche can't escape it, what hope do we mere mortals have?

Then again, if Binoche's Maria can ultimately shrug it off, so can we. Clouds of Sils Maria emerges into a meadow of optimism, but not before winding through some rather dark mountain passes. The “clouds” of the title refer to a meteorological phenomenon that unfolds, when the conditions are just right, along the Maloja Pass in the Swiss Alps: Mist thickens and gathers until it pours through the mountain fissures in a formation that resembles a slithering snake, an augur of heavy weather to come. To catch sight of the Maloja snake — and Assayas and cinematographer Yorick Le Saux capture it here — is to see something ephemeral but also spectacular. Nearly all of Assayas' movies are about capturing fleeting moments of beauty or magnificence, but Clouds of Sils Maria makes the metaphor even more literal: This snake, just made of air and water, nonetheless looks like something you can touch, an illusion worth believing in.

Maria herself has made a solid career out of make-believe: When we meet her, she's on a train wending its way through the European countryside, en route to Switzerland for a tribute to her mentor, playwright-director Wilhelm Melchior, who gave her a career-launching role at age 18 (in a play called, incidentally, Maloja Snake). Maria's assistant, Valentine (Kristen Stewart), is right by her side but also, it seems, in her hair. Valentine is on top of everything, every minute, answering her ever-ringing cellphone with clipped precision. Her manner, like the slouchy hoodies she's fond of wearing, is casual while still professional; she's flaky-sharp, in the way of so many millennials.

Yet Maria seems exasperated with Valentine, or perhaps she's just exasperated, period. She's in the midst of a divorce, and she's hoping to get back to playing roles in something other than the superhero vehicles she's recently been relegated to. On top of it all, she's got to write a speech in honor of a man she reveres. Then Valentine gets a phone call: Wilhelm has died suddenly; the tribute will be a posthumous one. Maria emerges from her own fog of distraction into a stunned kind of grief. What's more, a hot-shot, cooler-than-thou theater director (Lars Eidinger, who seems to be channeling Mike Myers' Sprockets character, perhaps unintentionally) wants her to appear in a revival of Wilhelm's play, about a young woman who seduces and destroys an older one — only this time, Maria won't be playing the ingénue.

As you're watching, Clouds of Sils Maria feels looser and sketchier than most of Assayas' other movies; only afterward do you look back and realize how many intricate layers he's packed into it. Chloë Grace Moretz shows up, as the Lindsay Lohan–style movie star set to play the role that Maria originally created; the dance between these two actresses is a two-step of calculated fawning and rivalry.

But the movie's true center, the meteorological phenomenon that makes it so pleasurable to watch, is the half-prickly, half-affectionate interplay between Binoche and Stewart. Maria is deeply dependent on Valentine, which may be why the older woman feels so comfortable snapping and picking at her. But in some ways, they're closer than siblings: In a beautifully tossed-off scene, they strip down — Valentine to a bra and white undies, Maria to the skin she was born in — and jump into a chilly Alpine lake, laughing and screaming along the way. Valentine runs Maria's lines with her, slipping all too neatly into the role of the younger woman who soon will nudge the older one into irrelevance — the tension between them hangs in the air, a silent crackle.

As relaxed and proficient a performer as Binoche is, Stewart is better here: She betrays her exasperation with her boss with little more than a slight eye roll or an exhausted shrug. With those coded gestures, Stewart shows us that Valentine is eager to get on with becoming something — but before that can happen, she needs to stop being an assistant. Maria never even asks Valentine what she might like to do with her life, probably because she doesn't want to know: After all, the moment Valentine moves on is the moment Maria loses her forever.

Clouds of Sils Maria may not cut as deeply as Assayas' movies tend to. But then, its greatest value may lie in the fact that it gives two fine actresses something intriguing to dive into. Their most radiant moment as a duo happens early on: They've finally arrived, exhausted, for Wilhelm's tribute. Valentine hasn't yet told Maria that she's scheduled to be fitted for a Chanel gown to wear to the gala. Earlier, we've seen Maria in her loose, comfy traveling clothes, her movie-star wattage turned way down. But once she's zipped into that dress — a sleek, dark column of silk — she takes on the air of a glamorous caryatid, a goddess of classical proportions. Spent as she is, Maria looks great, and she knows it — it shows in her panther-lady strut, her camera-courting smile. As she takes the stage to make her presentation, Valentine watches from the wings, shaking her head and laughing at the ridiculous wonder of the rejuvenated creature before her. She whistles, as if she were Maria's biggest fan — but then, that's exactly what she is. She has seen the reality behind the magnificent illusion, and knows there's so much more to it than air and water.

CLOUDS OF SILS MARIA | Written and directed by Olivier Assayas | Sundance Selects

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