If all the world's a stage, then you couldn't ask for a more dramatic backdrop than the Maloja Pass, a breathtaking swatch of the Swiss Alps where the light is clean, the air is clear, and clouds roll in, famously, like a snake writhing its way over the footprint of lakes nestled between sloping rock and trees.

This is where Olivier Assayas has set much of his hugely affecting - and reflective and witty - Clouds of Sils Maria, planting his two stars, Juliette Binoche and Kristen Stewart, in the titular town, isolated from the buzzing celebrity chatter of the outside world.

But celebrity chatter is very much on these women's minds: Binoche is Maria Enders, an actress in midlife, and, she hopes, in mid-career, still in demand for movies, fashion shoots, and lucrative commercials in far-off lands.

Stewart is Maria's personal assistant, Val, working a pair of smartphones and dozens of requests, appointments, hotels, and flight bookings, tricky social obligations. She is half Maria's age, and far more than a gofer.

They know intimate details of each other's lives. They argue the merits of books and films and filmmakers, trading show-biz gossip even as Maria professes her disapproval. She has played that game (in her first Hollywood film, her leading man was Harrison Ford) and perhaps she is still playing it. But not now, not here.

In the luminous mountain country, in a house belonging to a famous playwright who had discovered her, Maria prepares for a new production of his play - the play that introduced her to the world. This time, though, she has agreed, albeit hesitantly, to take the role of the older woman, an executive seduced and abandoned by an intense and charismatic new employee. Jo-Ann Ellis (Chloë Grace Moretz), an actress with a sci-fi franchise and a talent for making Lindsay Lohan-like headlines in the tabloid media, has been cast in the role that launched Maria's career.

As Maria rehearses her lines, with Val reading the part of the ingenue, the actress' past is literally staring her in the face. What does it mean to no longer have the gleaming allure of youth? To be playing the character left spent by the vibrant beauty who blows into her life?

Inevitably, though, as Maria and Val trade lines - and trade their interpretations of those lines - their relationship begins to mirror that of their characters.

Clouds of Sils Maria has the intensity - and some of the amped-up artifice - of live theater. Binoche, with her sad-eyed elegance (and those startlingly hearty guffaws!), inhabits Maria with a sense of familiarity. She and Assayas go way back (he wrote Rendez-vous, the 1985 drama for which she won the best actress prize at Cannes), and he clearly wrote this part with her in mind.

If it's cliched to say that someone's performance is a revelation, so be it. Stewart, who ran with vampires and werewolves and all that default teen angst in the Twilight franchise, bristles with intelligence, humor, and hurt. There isn't a moment, anywhere, when her Val doesn't feel entirely real.

In Clouds of Sils Maria, Assayas has created a work about the life of the theater (and screen) and the theater of life. It is about memory, about looking back from different vantages - from the quiet of a hotel room, from the dais of an awards ceremony, from an alpine crest, the sun going down, the mist coming in.

Clouds of Sils Maria ***1/2 (Out of four stars)

Directed by Olivier Assayas. With Juliette Binoche, Kristen Stewart, Chloë Grace Moretz, Johnny Flynn. Distributed by IFC Films.

Running time: 2 hours, 4 mins.

Parent's guide: R (adult themes).

Playing at: Ritz Five.EndText