Once-spidermonkey Kristen Stewart has swiftly escaped the confines of franchise cinema to prove herself one of the most promising young actresses of her generation. Her sullenness – relentlessly mocked by the tabloid press when she was at the centre of pre-teen Twilight attention – has become an asset in the subsequent years; as a performer, her work is defined by a reserve and intelligence rare amongst her peers. These are on full display in two recent films: Clouds of Sils Maria, out now in Aussie cinemas, for which she became the first American actress to win a César (basically a French Oscar); and Camp X-Ray, out now on DVD.

Clouds of Sils Maria casts Stewart as the personal assistant to Maria (Juliette Binoche), a famous actress returning to the play that made her famous, and spends its time primarily – you guessed it – amongst the clouds of Sils Maria (somewhere in the Alps, apparently. It’s very pretty). Camp X-Ray, meanwhile, occurs in the cramped quarters of a Guantanamo Bay detention centre (the film takes its name from a real prison camp), with Stewart playing one Private First Class Amy Cole and swiftly forming a complex bond with raucous/charismatic “detainee” Ali Amir (Payman Maadi). These are very different movies for the most part, but each primarily operates as a two-hander – Binoche and Stewart, Maadi and Stewart – and as a showcase for the considerable talents of these actors.

Clouds of Sils Maria is the second Olivier Assayas film I’ve seen, after After May (aka Something in the Air). This is the better film, but his talent as a visual stylist is undeniable in each. Sils Maria is chaptered into five sections (including an overlong “epilogue” that outlasts its precursors), and there’s a subtle pleasure to be found in the gentle formal shifts between each phase. Aside from the epilogue, none are explicitly identified as separate chapters, but languorous long shots of the Alps and aesthetic alterations are there if you’re looking for them.

The chilly orange-and-cyan palette of the first section is subsumed by the warmer, natural colours of the second: golden wheat, soft greens, the white of fresh-farmed milk. When the power dynamic briefly shifts between Maria and Stewart’s character, Val, it’s signalled both extravagantly – by a superb, cross-faded panic attack set to Primal Scream – and subtly, with Maria assuming her assistant’s pose as an observer, an outsider, as she gazes upon Val’s undressed form. (Maybe I’m reading too much into such framing decisions, but the importance of positioning seems to suit Val’s later comment: “The text is like an object. It’s going to change perspective based on where you’re standing.”)

The film brushes up against greatness, but never warrants the kind of comparisons Assayas seems to be striving for. Oh, sure, it has the ambiguity of Antonioni, the metatextual resonance of Kiarostami and intellectual dialogue centring on the philosophical purpose of art and so forth of Bergman. But its insistence on drifting effortlessly through its narrative (despite the chapters suggesting a rigid narrative structure that never eventuates) leaves it feeling bereft of purpose – it’s pretty and thought-provoking and interesting but never quite masterful. The final product’s reflections on aging and acting feel familiar rather than revelatory. That’s a minor complaint, given the immense pleasures found in watching Binoche and Stewart negotiate their roles and relationships in the film.

Stewart is introduced as the harried shadow of Binoche, juggling multiple mobile phones during a wobbly train ride to Sils Maria. But the relationship between the two women has infinitely more shades than beleaguered assistant and haughty superstar, and we’re quickly made aware of an unspoken intimacy bonding them together (not specifically romantic, though that’s not necessarily precluded). There’s a sense of humour and a spark of intelligence to Stewart’s performance. She combines an adolescent attitude – an aura of supreme nonchalance – with the ability to interrogate art and emotion with real intellect.

She’s the kind of person who can demonstrate a fondness for commercial sci-fi films and celebrity gossip – gossip that has more than a little intersection with the kind of snarky TMZ press directed at KStew herself – without becoming a airheaded caricature. The role asks a lot of Stewart – particularly as the bond between assistant and actor deepens and the line between reality and fiction blurs – but she delivers on every level, underplaying masterfully. Binoche is great too – obviously ­– but the fact that your attention is drawn to Stewart more often than not suggests the magnitude of her achievement here. It’s perhaps no surprise that the film begins to lose your interest when the focus shifts to Binoche’s rival, ingénue Jo-Ann (Chloë Grace Moretz), an up-and-coming young star whose career has a more than a few parallels with Stewart.

While Clouds of Sils Maria drifts past an underlying sense of rich complexity without ever really delving beneath the vaporous layers, Camp X-Ray approaches its material with a directness right from its opening frames. The film has an explicit agenda – to find humanity within the modern locus of man’s inhumanity to man: Guantanamo Bay. This isn’t the hoods-and-waterboarding era of imprisonment, mind you – the Army’s job is explicitly to keep the prisoners healthy and alive, not to torture them – but when Cole (Stewart) explains that they describe their wars as “detainees” because detainees aren’t subject to the Geneva Convention, the context is made very clear.

After an inauspicious introduction to bellicose prisoner Ali (Maadi) – which swerves from Harry Potter discussion to a “shit cocktail” remarkably rapidly – Cole forms a close bond with him. Like Sils Maria, the director – in this case Peter Sattler, helming his first feature film – sketches a symbiotic connection between the two leads. Unlike Sils Maria, Camp X-Ray makes the similarity between the two impossible to ignore, highlighting the similarities between Cole’s difficulty asserting herself as both a woman and a just human in an unjust system, and Ali’s imprisonment within a system that sees him as less than human.

That bluntness is probably overstated; a montage that goes out of its way to frame Cole and Ali in similar ways is unnecessary overemphasis. And for a film with such a blatantly political premise, Camp X-Ray is almost cowardly when it comes to politics. Cole’s victimisation at the hands of her thickheaded male colleagues is fleeting rather than systematic. Meanwhile the film goes out of its way to emphasis Ali’s innocence lest we inadvertently find ourselves sympathising with a possible terrorist. But, again, these failings are overshadowed by the thespian prowess on display (and a climactic needle-drop of the greatest track of the past decade).

In Sils Maria, Stewart inhabits her role effortlessly, bringing a warm, restrained humanity to her character. She demonstrates her prowess in dialogue; the way she reads lines with just enough authenticity and just enough remove. In Camp X-Ray, the discussions between Stewart and Maadi (who I can comfortably say is an outstanding actor, based on this and his work in Asghar Farhadi’s films) are riveting, but it’s the moments where Stewart says nothing – when she withdraws into her cell with a hint of the fundamental pain she feels – that truly resonate. She’s been widely mocked for her ever-present scowl, but there’s some truth to that sort of childish mockery – she really can find a depth of emotion with only minimal emoting.

Camp X-Ray doesn’t quite achieve greatness, either, but taken in concert with Clouds of Sils Maria it confirms that Stewart is an actress to watch. It’s hard not to.

Clouds of Sils Maria Rating:Camp X-Ray Rating: