Juliette Binoche makes her directing debut with In-I in Motion, a documentary about her 2008 collaboration with the British dancer Akram Khan on the dance-theatre love story In-I, which premiered in London to mixed reviews.

The film is, in essence, a study of their artistic process — and an unexpectedly fascinating one. It shows two world-famous performers pushing themselves outside their comfort zones, exposed and vulnerable. Binoche, in particular, had never danced professionally before the collaboration.

Like a wedding rehearsal, where the untidy feelings that will be tucked away for the cameras are still on show, the documentary feels raw and unguarded. It goes behind the scenes of the months of studio work it took to create the piece, capturing the endless stewing and fretting over how to achieve something truthful.

In-I in Motion is a film of two halves. The first, shot fly-on-the-wall style during the studio months, is the more interesting. Binoche and Khan work with the acting coach Susan Batson, who at one point thunders: "THIS IS FULL OF SHIT!" — never mind La Binoche's Oscar.

The second half ends with a performance on stage in front of an audience, on a set designed by the artist Anish Kapoor: a vast screen in murderous red.

For some audiences, the film will have a whiff of theatrical cringe; the endless fretting is ripe for spoof. But the pure intensity and focus of the two performers is thrilling to watch.

Eighteen years on from the mixed reception of the stage premiere, Binoche's film reframes the collaboration as a record of process rather than product — a rare, unvarnished look at what it costs two stars at the top of their fields to start again from zero.