'Her first orgy? I couldn't face it'

I don't know what your relationship to pornography is but mine - let us try to be candid from the start - is an unsuccessful emulsion of curiosity and bad conscience.

I don't know what your relationship to pornography is but mine ­ let us try to be candid from the start ­ is an unsuccessful emulsion of curiosity and bad conscience. I wouldn't dream of buying it, naturally ­ I'm far too cowardly to make that kind of commitment ­ but then I wouldn't avert my eyes either, should I happen to stumble across some. And this makes me ­ along with several million others ­ the perfect target audience for the porn documentary, that licensed peepshow that allows broadcasters to squeeze the sexually provocative through the narrow apertures of broadcasting regulations. This isn't a lap-dancing show, you understand, it's a serious consideration of the lap-dancing phenomenon which ­ for the purposes of sociological clarity alone ­ requires several unblinking close-ups of lap-dancers at work.

At first sight, Hardcore, a documentary about a young woman trying to break into the porn industry in Los Angeles, would seem to be just the latest contribution to this useful television genre. It turns out to be something quite different ­ a documentary in which the innocence of the subject and the innocence of the film-maker crumble in tandem in front of your eyes. The result is both explicit and numbing ­ a film about commercial titillation which ultimately leaves you limp with disgust.

It is not, by his own account, the film that Stephen Walker set out to make ­ even though he knew right from the start that he didn't want to replicate the prurient tourism of most porn documentaries. "At one point I wanted to make a film that would have no pornography in it at all," he explains. "I thought it would be a quite interesting challenge." Not a challenge that many broadcasters would be prepared to fund, though.

He was originally commissioned to make a film about a male porn star, hesitated over the obviousness of the subject matter, and then heard about Felicity ­ a single mother from Canvey Island who had been invited to Los Angeles by a porn agent called Richard, someone Walker had come across in previous research. Intrigued by the conflicts between motherhood and a career in porn, Walker agreed with his commissioning editor Stephen Lambert (with whom he'd worked on Modern Times) to take a chance with three days' filming. "I jumped on that plane going out to LA ­ the very same day that she was going, thinking I would be home 24 hours later. I arrived there, waited about four hours at the airport, met Richard and then she turned up. The film essentially starts with the very first frame I shot."

After making a series of documentaries about dark and difficult subjects in the early Nineties (including films about the Sabra and Chatila massacres and Mengele's experiments on twins), Walker had self-consciously opted for lighter subject matter in his recent work. His last two documentaries had been Jewish Wedding, about a mixed marriage, and Waiting for Harvey, a light-hearted account of film-makers trying to network at the Cannes film festival.

Hardcore wasn't intended to be a return to the underbelly of human experience. Walker's backers wanted something relatively light and his own attitude to the porn industry wasn't weighed down by moral disapproval. "I've bought a porn film and I've watched porn films before ­ it would be absurd to say I hadn't... Interestingly enough, since making the film I've watched no pornography at all. But I had, and if I'm going to be totally honest, I was really curious to know what it was like to be on a porn set."

He soon found out. "The fact was that I hated it. I found it depressing and numbing and totally unerotic. I was prepared to find that I found it extremely erotic and quite interesting. But the truth is that having been on about 26 porn sets, it felt like being in the bottom of a lavatory bowl after a while." The metaphor isn't entirely surprising, given the first job that Felicity secures ­ a session of still photographs which required her to straddle a bucket and pee for the camera ­ or the almost obsessional emphasis on anal sex among her potential employers.

What might notionally have been an intriguing spectacle quickly became a depressing obligation: "I can remember that we were supposed to be filming an orgy, an orgy which Felicity was participating in and it was going to be her first anal sex scene, so it was supposed to be an element in the telling of the story... and I just couldn't face it. It was on day seven and I turned to my recordist and my assistant producer and said, 'Let's go somewhere.' So we played truant. We went off and found an Italian restaurant and sat there with a bottle of wine and I felt bad that I wasn't filming an orgy!"

Having become friendly with Felicity and filmed her talking to her daughter in England, Walker was already feeling the pinch of his opposed instincts as a documentary maker and a human being. When I put it to him that every bad thing that happened to her made his documentary better, he concedes the point ruefully.

"It's true, in an awful, sick, terrible way. It's a documentary maker's dream. But there's a cost to that dream, there really is." It can't have helped that he often found himself using identical equipment to the pornographers, and was once mistaken for a crew member by a porn director, who asked him to secure the sort of close-up that he was at some pains to avoid.

The cost is clearest in Hardcore's most intense sequence, which record Felicity's unwilling encounter with Max Hardcore, a notoriously extreme porn star. After reluctantly agreeing to film with him (and ignoring Walker's insistence, audible on the soundtrack, that she doesn't have to), Felicity breaks off in tears, having been deliberately choked during an oral sex sequence. Walker found himself filming Hardcore's attempt to persuade her to resume filming.

"It's really easy, in retrospect, to look at something like that and say, 'Oh, you should just get in there and clobber the guy.' It's actually not as easy as that. A number of conflicting things are happening at that point: at a very basic level you're getting an amazing story on film ­ let's be honest about that, you are. At another level, you want to record just what really happens behind the scenes. You know that it's absolutely the kernel of your story and it's criminal not to film it. At the professional level, you're also in a house which is in the middle of nowhere, there are almost certainly guns around the place and there's this guy who I think is actually psychopathic and you're scared..."

Eventually, when his assistant producer Tessa Gogol pointed out that they were effectively about to film a rape, Walker stepped in, persuading Felicity not to continue and escorting her from the house. "I double-locked all the doors to the hotel room and put the rushes underneath my bed, and about three o'clock in the morning I rang my wife and ­ this sounds terribly silly ­ I completely broke down, I was in floods of tears. I kept saying to my wife, 'I don't know whether I did the right thing...' Thank God I did it in the end ­ but I'm not at all sure that it was not too late."

Only a few days later, Felicity agreed to take part in another extreme scene. Walker walked out after three minutes, by which time one performer was bleeding. It was at this session that the director ­ who refused to have his face filmed ­ boasted to Walker that he had offcuts from his films that made "Belsen look like a picnic".

By and large, the film doesn't make its excuses and leave ­ the classic get-out for prurient documentation. Instead, it stays and chooses to look at the faces of those involved in this business. And it offers something else besides a frightening slide into moral vacancy.

After returning to England, Walker went to talk to Felicity again and recorded a long conversation about her feelings for her natural father, with whom she'd been re-united in her teenage years but who had failed to keep up the contact. Her most nakedly revealing moments are not those in which she performs sex-acts for money (Walker largely avoids filming her body), but her stumbling explanation that her reckless self-exposure is a form of revenge against her absent father.

Three days after that interview was filmed, Walker says, she rang to tell him that she had given up working in porn. Though both Walker and Channel Four say they were prepared to pull the film if she didn't want it transmitted, she insisted that it should go out. It's very difficult to say whether she is right or wrong in that decision ­ having proved so wildly erratic in her judgement about cameras before ­ but Hardcore is not a film that will leave any illusions about the nature of porn undisturbed. It's worth turning on ­ just don't expect it to return the favour.

'Hardcore' is on Channel 4 tomorrow at 10.35pm