Circle of friends: "Billie Eilish: Hit Me Hard and Soft"

It is, all told, an unlikely partnership. Before the camera: Billie Eilish, the recessive songstrel with the million-dollar mumble. Behind it: James Cameron, the Hollywood alpha dog who should really be back in his shed getting going on the next Avatar movie. Well, maybe the live concert movie Hit Me Hard and Soft is Jim Cameron giving himself a day (or a night) off; maybe it's his way of bonding with his daughters after several long years at sea, or maybe his wife Suzy Amis has told him, in no uncertain terms, that he needed a hobby that doesn't involve blue people. (The movie has been conceived in 3D, so it's not a total break from recent endeavours.) The question cinephiles face is whether we can tell this is a concert movie assembled by one of the great motion picture technicians, and whether Cameron has more of an idea of what he wants to film and show than whoever it was who did the Taylor Swift concert movie, which emerged as all coverage and nothing but coverage. For starters, Cameron demonstrates an inevitable interest in the tech of a tour such as this: the rigging going up, the sea of phones capturing moments, the catapult that propels the singer on stage at a crucial moment. He also engineers several time shifts one might, at a push, describe as Terminator-like ("8 hours before the concert", "Forty minutes earlier", etc), the better to chronicle how this particular show came together at what feels like the last minute. This may just be Cameron, notionally playing second fiddle for the first time since he made 1982's Piranha II: The Spawning for Roger Corman, ceding a certain degree of his usual control. Some of that control is ceded to the crowd, who certainly weren't here to play extras in a Jim Cameron movie; their heads and arms pierce the frame (somewhat joltingly, an effect that tends to be elided from slicker 3D spectacles) while their screams and shouts frequently take over the soundtrack. Most of that control, however, is ceded to Eilish herself: Cameron has even left handheld 3D cameras around the stage for her to pick up and run with in the course of the concert. The results come to seem like a true collaboration, with caveats. "It's going to say 'Directed by Billie Eilish'," Cameron is heard to say backstage at one point, "and then - at the very bottom - 'with James Cameron'". As it happens, the closing credits actually read "Directed by James Cameron and Billie Eilish". Ah well.

In terms of the show itself, the Eilish mumble appears central to her relatability: she is, from the off, far more approachable than La Swift, the Business Barbie with the celebrity athlete husband and the preternatural gift for songwriting. Swift, certainly, wasn't likely to allow a director to film her having her ankles strapped up backstage, or going through a rehearsal session with her vocal coach, or chatting while applying her pre-gig contouring. ("It really reads from a distance," notes Cameron, in full 'cool dad' mode.) Her stage outfit - just the one, unlike Swift's six billion - is a lightly worn shrug: loose sports jersey, backward-turned baseball cap, Limp Bizkit shorts, glasses apparently sourced from the venue's lost-and-found box. It's not even smart casual, and yet over the course of this concert - liberated to move in any which direction, both physically and musically - Eilish becomes a recognisably Cameronian figure of interest: a woman who comes to command an army of diehard followers while reshaping the fabric of time and space. Non-diehards (and here your correspondent must include himself) might want a little more variation in the songcraft, gazillion-sellers though these tracks may be, beloved though they visibly are of this crowd, captured trilling along with tears in their eyes. (Cameron catches so much saltwater I wondered if Hit Me Hard and Soft was going to function as an Avatar origin story: this is how whole planets flood.) And the tropes of these all-new concert movies are now such one senses the Documentary Now! lads in the wings, preparing a bumper double episode: they could have a field day with the 'puppy room' Eilish insists on having backstage so as to reduce pre-gig stress ("everyone needs some dog love"). None of this matters, though, so long as Cameron films his subject with much the same awestruck gaze as he once did Sarah Connor and Ellen Ripley. Part of the filmmaker's fascination, I think, is that Eilish has successfully accomplished something he hasn't: to downsize. When Eilish sits crosslegged on her comparatively no-frills stage ("I don't want anything between me and them", she tells her director) and hushes the crowd into total silence before one song, she shrinks a cavernous concert venue to a small, tight friendship circle, as safe a space for creation as her own teenage bedroom. Eilish gets Cameron to think small for the first time in decades: the result is a rare concert movie that converts the colossal spectacle of the internationally touring pop show into something personal, intimate and very charming.

Billie Eilish: Hit Me Hard and Soft is now playing in cinemas nationwide.