Sex scenes: In the realm of the senses...

What makes a film truly erotic? Unrequited longing, transgression, voyeurism? Can men and women ever agree? And why are film polls on the subject always so disappointing? Channel 4's 100 Greatest Sexy Moments was a prime example. The Top 10 ended up a mix of soft-core classics (Basic Instinct, Emmanuelle, Nine 1/2 Weeks) and films that treat sex as slapstick (American Pie, Who Framed Roger Rabbit?). There was very little grown-up discussion of the co-ordinates of desire.

Which is why director Sophie Fiennes' latest project, The Pervert's Guide to Cinema, which is showing at selected screenings around the country, is so refreshing. In her trilogy, the cult philosopher and psychoanalyst Slavoj Zizek takes us on a brilliant and unhinged road-trip through some of the greatest movies ever - from Hitchcock's romantic epic Vertigo, to the films of David Lynch and Stanley Kubrick's Eyes Wide Shut, delving into the hidden language of cinema, and uncovering what movies can tell us about ourselves. Key to The Pervert's Guide is an exploration of the relationship between desire and fantasy in film.

"Today in film financing," says Fiennes, "there's this obsession about plot and about scripts and about a story and 'would a character do this?'. It's making cinema very thin in terms of the density of feeling that it can generate." For Fiennes, the critical factor is in how a film's form delivers its narrative, because the mind/body relationship to the moving image is extremely complex. "I agree with Zizek when he says the true cinematic pleasure resides in the 'libidinal density of cinematic form itself', and it's actually very mysterious. When you go to a David Lynch film, you feel that uncomfortable moment. Something happens where it's like... woaah. There's an intersection of elements conspiring to make it a moment that's memorable or disturbing, and it's often beyond the basic narrative."

Viewed through a psychoanalytic lens, the movies she and Zizek analyse in The Pervert's Guide become far more arousing than pornography precisely because we see so little flesh, and because we see how subtly these films play with our desire. The Pervert's Guide uncovers cinema's ability to speak to our dark libidinous urges. In part one, Zizek tells us, "Cinema is the ultimate pervert art. It doesn't give you what you desire - it tells you how to desire." Film arouses our desire while simultaneously "keeping it at a safe distance, domesticating it, rendering it palatable".

Zizek is a riveting presenter. He never talks down to his audience - explaining the most complex Lacanian/ Marxist theories through Hollywood film clips. In fact, he is so compelling it's easy to forget the films were directed at all. But Fiennes is the master puppeteer behind the scenes. It was her decision to flesh out his monologues by shooting at original Hollywood locations and on replica sets - such as the Bates Motel in Psycho, or getting Zizek to steer a boat through Bodega Bay as Tippi Hedren did in The Birds. This gives us the illusion that Zizek is speaking from within the films themselves.

Even the film's sexy title is part of the tease. "The film's title is just a way to get you into this Zizekian network," laughs Fiennes. "But, yes, cinema is a kind of ritual through which we expect to experience sensations: it physically affects us and in this sense what we find in the cinema is disturbingly real, not all that fictional. The most extreme example, of course, is pornography; there you can measure the effect at its crudest."

The great test of many of the films featured in The Pervert's Guide is that they stand up to repeated viewing. "You can measure your development psychically against certain films: they almost become like boxes containing part of your psyche," says Fiennes. From Lynch's Lost Highway and Ingmar Bergman's Persona to Michael Haneke's The Piano Teacher, the clips in The Pervert's Guide demonstrate how fantasy is the battleground of the war between the sexes. Zizek argues that it is the very excess of female desire that poses a threat to male identity. Vertigo emerges as the most damning example of them all, where "for the male libidinal economy," Zizek states, "the only good woman is a dead woman". In fact, in Vertigo, Scottie (James Stewart) doesn't want a flesh-and-blood sexual partner, preferring to "recreate" a fictional woman (who is dead, but actually never existed). The only way Judy, played by Kim Novak, can have sex with him is to submit to his fantasy.

Sometimes paranormal events in a film can represent forbidden sexuality. For Zizek, Hitchcock's The Birds is all about incestuous tension: Mitch's mother (who looks uncannily like an older version of his girlfriend) is threatened by her son's relationship with the blonde outsider, Tippi Hedren, so that the attacks of the birds are "explosive outputs of maternal super-ego". "But the brilliant twist here," says Fiennes, "is that this maternal superego is actually the male fantasy, it is Mitch's own incestuous internal conflict, not his mother's. Like poor Norman Bates in Psycho."

Inevitably most of the directors featured in The Pervert's Guide are male (historically, they have had greater access to the means of production). Which is why Fiennes was so determined to include the no-holds seduction scene from In The Cut, where Mark Ruffalo tells Meg Ryan, "I can be whatever you want me to be. You want me to romance you, take you to a classy restaurant... if you want me to be your best friend and fuck you, no problem."

It is a political moment, she argues. "In making this film, it was quite tragic that while Zizek is arguing for women having this amazing capacity to fantasise, these fantasies seem to be strangely absent, as it were unpublished, invisible." A point that is backed up by director Kimberly Peirce in the current documentary This Film Is Not Yet Rated - where she reveals that Boys Don't Cry got an N-17 rating because Chloë Sevigny's orgasm scene was considered too visceral.

As for the sex scenes in In The Cut - "a film that is brilliantly interesting for us women - was just ridiculed when it first came out," says Fiennes. "It was demonised and erased. Male critics hated the Mark Ruffalo character, but if you ask women they have quite a different response. Making The Pervert's Guide I started to get the overwhelming sensation that men and women don't actually see or hear the same thing when they watch a film. It is what psychoanalysis calls 'sexual difference'."

For Fiennes, film works like alcohol: it gives you Dutch courage to take on the chaotic nature of life, to help you plot the co-ordinates of your desire for better or for worse. The Pervert's Guide To Cinema targets the brain and the libido - it's a great date movie.

'The Pervert's Guide to Cinema: Parts 1, 2 & 3' is released on DVD on 22 November

www.thepervertsguide.com