A pain in the arse would be one way of describing the films of Jacques Rivette. Very few of them are much shorter than three hours and most of them last nearer four. It would take buttocks more buttressed than mine to sit entirely comfortably for that kind of length of time. Not to mention Out One: noli me tangere, which clocks in at a full twelve and a half hours.

Granted, Rivette helpfully broke that particular epic down into eight rather more manageable episodes. Even so, few people have ever managed a full viewing. Prior to its screening in the National Film Theatre's current season of Rivette pictures, Out One has been shown precisely once, at Le Havre in 1971. Even French TV spurned the chance to air the film - which is one reason the director re-edited his miles of celluloid into a film I always think of as Out Two, but which is actually called Out One: spectre (1972). Hold your horses, though, if you're thinking Spectre might be the easier option. Radically recut as this offspring is, it still chalks up a minatory 270 minutes' running time. (Plus, as the NFT programme thoughtfully notes, interval.)

La Belle noiseuse (1991), perhaps Rivette's best-known picture thanks to Émmanuelle Béart strutting through most of it in the buff, is almost as long. It even parodies the audience's pains by having Michel Piccoli, playing a quietly brutish painter, stretch and squeeze his model (Béart) into all manner of purgatorial positions. Unsatisfied as ever, Rivette recut the film, this time using footage not even seen in the original, and ended up a couple of years later with something not much over two hours. Brazenly, he retitled it Divertimento, as if this new-found brevity made for an entertainment of Mozartian lightness.

In a way, he was right. Long though even Rivette's shortest films are, laden with longueurs they are not. A case might be made that Paris nous appartient (1961), for all its slow-burning Magrittean paranoia, doesn't quite justify its 140 minutes. But there are two reasons for it.

First, this was Rivette's debut feature, and he freely admitted to having learned on the job. ("I am very unhappy about the dialogue," he confessed years later. "I find it atrocious.") Second, the film was shot piecemeal, and for peanuts, over a period of more than two years. If the actors look bored with their roles, that's probably because they are.

Rivette being Rivette, however, he factored this boredom into what I hesitate to call his storyline. Paris nous appartient isn't just acted by a cast that would rather be doing other things. It is also about a group of actors who would rather be doing other things. In the film, their boredom is induced by an improvisational production of Pericles.

This is a way of admitting that he is that perennially unfashionable figure, the modernist film-maker. Not for him the rising character arc, much less the three-act redemptive drama. John Ford, Nicholas Ray and Fritz Lang were the young Rivette's first cinematic loves. Yet, unlike his fellow former critics-turned-directors whom we identify with the Nouvelle Vague (Jean-Luc Godard, Claude Chabrol, François Truffaut), he never attempted to follow in his heroes' footsteps. While Chabrol has spent most of his career imitating the Langian thriller, and Godard has wasted too much of his creative life subverting the diktats of the musical, the western and the gangster flick, Rivette has ploughed a lonelier furrow. He looks sui generis because he acted as if Godard's generic diktats didn't exist.

Secret defénse (1998), with its girl scientist Sylvie (Sandrine Bonnaire) trekking across France on the hunt for her father's murderer, might sound like something out of Hitchcock, but it comes across like something out of Heidegger. Here is another long film, at almost three hours, chock-full of the mundane and the quotidian, the only real drama arising from the long, steady shots of Sylvie's face, pulsing with ache and dread as she arrives, with thrilling slowness, at her decisions.

There are also speedier thrills to be had in the magnificent season at the NFT. Say hello to the re-release of Rivette's hil- arious and, yes, moving Céline and Julie Go Boating (1974), a slapstick comedy cum surreal thriller cum love letter to Paris, in which our titular heroines become involved in the cyclical shenanigans of a haunted house. Or are they just bombed out of their brains and making it all up as they go along?

One thing is for sure: the actors playing Céline and Julie - Juliet Berto and Dominique Labourier - really are making it all up. They improvised their dialogue in front of the camera with no more than the odd hint from Rivette. Unsurprisingly, they look like genuine big-screen life forces of a kind not seen since The Big Sleep (1946) by Howard Hawks, another of Rivette's early heroes. Watch Berto and Labourier in action, glee-fully overthrowing patriarchal rule, and then try telling me that Jodie Foster is a feminist movie star.

But then Rivette has always been a wonderful director of women. For all the avowed radicalism of early Godard - his desire to overthrow narrative norms and bourgeois mores - there is something mighty conventional about the eye-candy treatment meted out to his then squeeze, Anna Karina. By contrast, Rivette's Diderot-inspired La Religieuse (1966) takes as its basic theme the idea that beauty will always have beastliness trailing in its wake. Karina plays a nun who, after being hit on by lesbians and a rapist priest, ends up working in a brothel.

Movies must be more than abstract arguments, and what really counts in La Religieuse - as in all of Rivette - is the looseness, the joyousness, of the acting. The more programmatic Godard could never have coaxed Karina to such Zen-like spontaneity.

It would be folly, however, to pretend that this retrospective will be easy going. Rivette's pictures were hard enough work four decades ago, when Hollywood itself was belatedly discovering modernity in movies such as Bonnie and Clyde, The Wild Bunch and Easy Rider. In our infantilised age, the era of The Da Vinci Code and Mission: impossible III, the likes of even Divertimento can seem like the cinematic equivalent of the eleven-plus. Yet not all pleasures are easily won, and the hardest are usually the most pleasurable. "I speak in riddles," says a character in Paris nous appartient, "but some things can only be told in riddles." Indeed - and some things can be worth suffering a pain in the arse for.

Céline and Julie Go Boating is re-released on 5 May. The Jacques Rivette season continues at the National Film Theatre, London SE1 (020 7928 3232) until the end of May

A brief history of long films

The Cure for Insomnia (1987) 5,220 minutes
A snooze-inducing, 87-hour montage of X-rated movies, rock videos and live poetry readings. Dir: John Henry Timmis

Burning of the Red Lotus Temple (1928) 1,620 minutes
A commander takes 27 hours to escape a booby-trapped temple in this silent Chinese adventure in 18 parts. Dir: Zhang Shichuan

The Journey (1987) 873 minutes
Spanning four continents over 14.5 hours, a relatively concise take on the nuclear threat. Dir: Peter Watkins

Research by Ally Carnwath