His latest film's explicit sex scenes were booed at Cannes but director Carlos Reygadas is not bothered about the opinions of others, he tells Donald Clarke.

The opening scene of Carlos Reygadas's astonishing new film, Battle in Heaven, finds a young woman, played by a non-professional actor, performing oral sex on an overweight gentleman, who later turns out to be an inept kidnapper.

The sequence, a reprise of which explicitly confirms that nothing is being faked, attracted quite a few boos at the Cannes Film Festival earlier this year. Reygadas, an overpoweringly self-confident 34-year-old Mexican, is, however, not the sort of fellow to offer apologies.

"Some people say that if you are actually doing this act then the film becomes a documentary," he says. "I just don't see that. I mean if you are actually eating a sandwich in a film, does that make it a documentary?"

He is surely being a tad disingenuous here. I don't imagine the punters in Cannes were booing because they objected to the blurring of the line between drama and documentary. I tell Carlos about a conversation I had with the Australian director Jane Campion concerning a similar scene in her film, In the Cut.

Campion explained that she could not ask a young unknown actor to perform a sexual act, because she could not be certain that such a performer wasn't just pretending to be comfortable with the scene in order to get the part. Ask an actor if they can ride a horse and, even if they have never been astride anything livelier than a tricycle, he or she will tend to say yes.

"I can discount that," he says forcefully. "It is completely ridiculous. Trained actors are even more desperate for parts than non-trained actors. They are more prepared for competition. That's their life. What is the difference between asking an actress to kiss or do this? Look, if we were making a film in Iran then even a normal shirt that a woman wears might be regarded as unacceptable. People would be outraged at seeing a woman's shoulders. It is purely a cultural distinction."

Oh come on. He did not make a film in Iran. He made a film in Mexico, a Catholic country, most of whose citizens would see a very clear distinction between kissing on screen and doing, well, this. "You are right. That is true," he says. "Okay. I care about my own world-view and what I myself think of the body. I am not going to carry out a survey to discover what others think before I make a film."

Phew! I don't imagine Carlos Reygadas's often welcomes self-doubt into his trailer. Born in Mexico City in 1971 to a father in arts administration and a mother who was once an anthropologist, then a Jungian psychologist, Carlos qualified first as a lawyer. He was working in the United Nations when the urge to create became overpowering. "I was doing interesting work and I enjoyed it, but there was an internal need within me to do something more powerful. I needed to make films, but I didn't immediately know that. In a different time I might have gone into the jungle like in Conrad or gone on an expedition to the North Pole."

With characteristic drive, Reygadas borrowed a camera and began shooting short films. His hugely impressive first feature, Japón - it's not set in Japan and features no mention of that country - told the story of a painter who retires to the wilderness to commit suicide. Shot for just $60,000 (€50,000), borrowed from art dealer buddies, the film won a bundle of awards and became a high-brow hit. "When they began applauding it at the Rotterdam Festival I thought they were doing so out of pity," he says in a rare moment of humility. "But they actually liked it."

Battle in Heaven marks a major step forward for the director. Whereas the earlier film, though hypnotically impressive, seemed a little too obviously indebted to the work of the Russian director Andrei Tarkovsky, the new picture throbs and buzzes (literally so - there are blaringly loud noises throughout) with new energies all its own.

We follow the affairs of a middle-aged couple who have botched an attempt to kidnap a neighbour's child. The unfortunate infant died shortly after being apprehended. Despite that scandalous opening scene, Battle in Heaven strikes me as a surprisingly moral film. The husband, Marcos, gradually becomes overcome with guilt and eventually gives himself to God. I suspect Reygadas's aesthetic is far too austere to accommodate anything so bourgeois as a message, but the picture does appear to be saying that we can never quite escape our most severe moral transgressions.

"I didn't intend it to be moral, but I do acknowledge that that might be true," he says, slightly reluctantly. "If we do go against our moral nature, eventually we may get into trouble. But Marcos's wife does find a way out through habit and custom - by just doing what everyone else does in their lives. I don't seek to judge either of them."

Reygadas points out that he felt no need to put the characters in serious financial difficulties to justify their decisions. He believes that a general moral decay has taken over Mexico since 1995 when former President Carlos Salinas was forced into exile after his brother was implicated in the murder of a political rival.

Again, this seems a surprisingly conventional approach: when leaders don't set a good example, their people may drift towards vice. "It's not that I think they should set a good example. But there can be a sort of empathy between particular people and their leaders. Imagine if Tony Blair murdered an ex-rival. I think people would feel able to become more violent. There would be more shooting and killing in Britain."

Reygadas does seem preternaturally unflappable. How did he get this way? He didn't go to film school, yet he seems enviably comfortable with the technical demands of movie directing. Did he learn by watching the masters on video? Did he lurk around other directors' sets?

"Everyone tries to convince you that it is very hard to make films, but it's not," he snaps. "It is very simple. It is like painting; it is easy to mix the paints, but to make good art from them is hard. All you have to do is set up the camera, push the button and make sure there is something interesting in front of it. Oh and then decide what will follow and what came before."

So there.

• Battle in Heaven opens at the IFI, Dublin, Friday