It was while watching the new film Intimacy, at about the point when the actress Kerry Fox takes her co-star Mark Rylance's angry-looking penis and pops it into her mouth, but before the bit where she gives it a few brisk, matronly rubs, that I started thinking about dead animals. This wasn't a reflection on the performers' physiques – it was reassuring, for once, to see bodies that had been moulded by life, rather than by scalpel or barbell. And it wasn't to curb any undue excitement either; when it comes to mental equivalents of a cold shower, I find listing Walter Matthau movies in chronological order beats them all. Have you ever got past The Taking of Pelham 123? I got up to Hopscotch once. Not that I'm boasting.

The reason that dead animals sprang to mind was that the experience of watching actors having sex, or at least foreplay, in a non-pornographic film was oddly reminiscent of that moment in Weekend (1968) when a chicken is killed before your eyes, or the scene in Benny's Video (1992) when a pig is dispatched with a bolt-gun. In these instances, you can feel yourself being dragged out of the movie's sealed universe by the realisation that this isn't a prop pig, and that's not Kensington gore. Once a narrative film spills over the borders of artifice – and it can be something as simple as the baby that is distressed by an on-screen argument in Guiltrip (1996) – the illusion is shattered and the film has relinquished its grasp on the viewer.

We like our actors to be blisteringly real. Give us a De Niro or a Day-Lewis and we'll throw all the acclaim in the world their way. But our obsession with authenticity has only increased as film-making has become more realistic, throwing up a volume of inquiries. Is Divine really munching on a turd in Pink Flamingoes? (yes); is little David Bennent actually watching his screen parents making love in The Tin Drum? (James Ferman, the chief censor back then, believed so, and forced Britain to become the only country to release a cut version of the movie); are Warhol's actors genuinely stoned? (take a wild guess). Then there are the questions of animal cruelty to which the BBFC must expose every film. Never Say Never Again? Guilty. Amores Perros? Not guilty, M'Lud.

In most of those cases, there is just enough ambiguity to prevent the act in question from disrupting the film's equilibrium. Screen sex can be more difficult for the viewer to manoeuvre around. The factors that undermine your concentration can be gossipy (what must his/her wife/husband think?) or it might simply be the incongruous presence of this intimate act in the context of the larger pretence of a movie.

With Intimacy the problems are pronounced because the actors are so well-known. One of the unspoken bonds that a performer makes with an audience is one of familiarity: a string of good performances earns a viewer's trust, and can result in someone seeing a movie solely because Kerry Fox or Mark Rylance or whoever is in it.

That feeling of connection or kinship is integral to the relationship between film actor and audience, and like any relationship it thrives on a degree of mystery which usually precludes even full-frontal nudity – hence the fuss when Bruce Willis cast Little Bruce in a cameo role in Color of Night (1994). Of course, Intimacy could have been even more disruptive for its viewers if the director Patrice Chéreau had landed his first choice for the lead. Thankfully, Gary Oldman declined, though he did go on to sacrifice his dignity in a whole different manner in that irritating One-2-One ad.

While a film-maker can forfeit the fiction that he or she has established when actual sex is introduced, the use of unknowns or non-professionals lowers the risk. The actors in La Vie de Jésus (1997) are obviously known to someone, but as far as the audience is concerned, they could have been picked off the street. In fact, they look like they've been picked out of the ground: no Hollywood grooming sessions or aerobics classes for these oddballs. So when the brief penetration shot arrives, it may be mildly shocking, but it's also consistent with the film's mood, which is grim and earthy and designed to make you shudder in your seat.

An equivalent scene in Pola X (1999) comes off less effectively because the actor enjoying a shadowy but graphic sex scene with his co-star is the relatively famous Guillaume Depardieu. Looking for clues to his perspective, and perhaps a jolly anecdote (... and then we found there was a hair in the gate!), I checked out the American DVD of Pola X, which features a commentary by Depardieu. But come the revealing shots, he mumbles a bit, remarks of his co-star "She, er, was a circus contortionist" and then keeps a respectful silence for the duration of the scene. Which perhaps says it all.

Lars von Trier's The Idiots, with its shots of erections and actual penetration, is one of the few films in which "real" sex proves crucial to the plot; those glimpses are important for signalling an abrupt change in the movie's tone, from tomfoolery to something darker. That point is also a sign for the heroine – and the audience – that it is time to pull back. And since the tension throughout the film emanates from the suggestion that it might all really be happening, there is no illusion for the sex scenes to disrupt; once you've seen the boom-mike poking conspicuously into shot, an erection or two isn't going to jar you out of your precarious make-believe world, because the film has denied you that comfort from the start.

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The release of The Idiots coincided with the beginning of the BBFC's liberal maturity, and to the casual observer it may have seemed like there were frank sex scenes being tossed into movies willy-nilly. Seul Contre Tous (1998) featured a long excerpt from a pornographic movie viewed by the main character to express his emotional desolation. That scene was subtly smudged by the BBFC; the meaning remained, if not the act itself. But erections abounded in Romance (1998) and one popped up in Sitcom (1998) just for good-natured shock value – the same reason that two characters in that film who bathe naked together were played by real-life siblings. Hard to believe that scarcely a decade earlier, there had been a scandal over Scandal (1989) when the censor spotted actual sex taking place in the background of an orgy scene.

With films such as Intimacy and Pola X, the inclusion of real sex becomes shorthand for letting the viewer know that the director has his fearless hat on today – that risks will be taken, rules broken. But revealing everything isn't the only way to do that. Gus Van Sant has crafted some innovative work against the odds: the itchy erotic heat of Mala Noche (1986) was entirely whipped up in the editing room after one of the actors refused to play a gay sex scene. And despite our country's reputation for prudishness, the examples of movie sex that have really made the earth move have come from a British director, Nicolas Roeg. Don't Look Now (1973) features a sex scene so convincing that it has long been dogged by rumours that its stars, Julie Christie and Donald Sutherland, were not really acting at all. The confusion is a compliment to what Roeg achieves on screen. That film, along with The Man Who Fell to Earth (1975) and Bad Timing (1979), testifies to his respect for sex in cinema as a means of communicating information about character and motive, rather than as spice or garnish.

I can believe that Chéreau set out with the same intention on Intimacy, but the candid sex in his film only accentuates an already uncomfortable tension between truth and illusion. Yes, the picture's South London locations look gritty, but the script has the feeling of having been translated through several languages before reaching English. In this atmosphere of awkwardness, the sex scenes are both truthful, and, paradoxically, a threat to this weird, otherworldly London where a gay French bartender dispenses wisdom on life and the supporting characters appear to be reading from phonetic cue cards.

Either way, the sex doesn't fit. Not that I'm being prim. Like many others, I misspent youthful afternoons at London's now defunct Scala Cinema watching Curt McDowell's 1975 camp classic Thundercrack! (you can always spot people who are slightly guilty about having enjoyed this hardcore sex comedy, because they call it a camp classic). Nothing in that shocking picture was faked. I bet even the scene where a woman's wig falls into the toilet while she's vomiting, only for her to retrieve it and replace it on her head, was for real.

But as a rule, real sex rarely enriches a movie. Better all round to make it "like a robe pontifical,/Ne'er seen but wonder'd at" – a line from Henry IV Part One, which you will recall has no erections or penetrations, and is none the worse for the lack thereof.

'Intimacy' is released on 27 July