Outside it's just another dull December day. But inside, away from prying eyes, Ralph Fiennes, arguably the greatest British actor of his generation, is doing unspeakable things to an imaginary woman. “I'll never forget it,” he says, half-kneeling on the floor of our sparse-but-swank hotel room, holding two imaginary legs apart, bending forward and pressing, into the imaginary birth canal of his imaginary paramour, his very real and freshly shorn head. “It's literally about trying to get inside her womb!” He chuckles fondly as he further enacts what can only be described as an attempted full-body reversed birth.

The re-enactment, he explains, is of a moment in the 1981 Ben Gazzara movie Tales of Ordinary Madness, in which “this unhappy man [Gazzara] is with a woman, and he literally wants to climb back inside her”.

On any other day this might have seemed like an odd skit to come from the 46-year-old star of The English Patient, Schindler's List and two Harry Potter movies (where he played the boy wizard's evil nemesis Voldemort). But today Fiennes is on fire. He's more than halfway through a mesmerising run of Sophocles' brutally unforgiving Oedipus at the National Theatre and is thus somehow fantastically engorged with life's Big Ideas.

“For me the lines in the play that resonate continually are, ‘I want to know the secret of my birth' and ‘I do not know who I am!'” he says, dropping his voice to a whisper and running a hand slowly over his shaven pate. The haircut was his idea, he explains, to make him feel more naked on stage, but it has a powerfully imposing effect that, together with today's outfit of blazer and denims with turn-ups, creates the impression of a patrician bouncer. “And isn't that the journey that most of us are on?” he continues, undaunted, transfixed by the tragic clarity of his words. “Who are we? What are we doing? And where did we come from?”

Questions, questions, questions. We're supposed to be talking about The Reader, an intriguing post-Second World War drama from the director Stephen Daldry (The Hours), in which Fiennes stars as a German lawyer reflecting on a doomed love affair, but for now the primal pull of Oedipus is simply too great.

The play is agony, he says. The play might destroy him yet, he adds. And the play, as we all know, is about mothers. It's all their fault. They are the site of the eternal return - that metaphorical place to which we, according to Sophocles, Freud and Fiennes, spend our life returning. We are trying, says Fiennes, to re-enter the womb (hence the skit). And that, he says, can sometimes get in the way of a decent romantic relationship.

“There is a tension in relationships between wanting to return to the womb, but also wanting to be free,” he says, with impressive candour for a man who was, until they split in 2006, often described as being in a vaguely “maternal” relationship with the actress Francesca Annis (17 years his senior). “Because sometimes the woman's attentions can be overly maternal, and you want to go, ‘Ahhhh!'”

The best thing, however, about Oedipus, says Fiennes, is that it is provocative, disturbing and difficult, the way that real theatre should be. “When theatre becomes a soothing middle-class thing, when it's packaged as the Night Out, then that's the death of it,” he says. When I say that most of his Oedipus audience left the performance that I saw in a shell-shocked depression he coos excitedly, “Good! Shake 'em up! Shake 'em up! Hahaha!”

We move on to The Reader, which has its own share of provocations and mother issues. These are in the erotic and Oedipalised relationship between Fiennes's younger screen self (played by David Kross) and an older, mysterious and illiterate former Nazi, Hanna Schmitz (Kate Winslet). The provocations, says Fiennes, are in the plot, “and the idea that Hanna's illiteracy could be an excuse for her joining the SS in the first place. I don't accept that she couldn't make a moral judgment about joining the SS just because she's illiterate. And I think that's a troubling area.”

His performance in the film is, typically, another impeccable study of inner turmoil. With his eyes alone, palely plaintive and wildly expressive, he gives the greatest close-ups in the business. It is perhaps worth noting that Fiennes onstage and Fiennes onscreen are two radically different beasts - one dominates all available space with masterful physicality, the other is the essence of stillness, and lives almost entirely in glances, looks and stares. And yet there is something of the reluctant movie star about Fiennes. “If I had a gun to my head and I had to choose between theatre and film I'd choose theatre,” he says, before qualifying that although he finds the whole business of stardom disturbing (the fans, the demands, the intrusiveness), he is nonetheless still hungry for juicy movie roles.

Elsewhere, since 1995 and his Tony award-winning performance in Hamlet, he can be read as a theatre actor who regularly tears up the transatlantic boards in plays such as Coriolanus, Richard II and The Faith Healer, and then drops into film with some devastating turns in The End of the Affair, Spider or The Constant Gardener, just to let you know how good he is.

And he is good. His theatrical peers speak of him with hushed reverence. The actor Simon Russell Beale has spoken of seeing Fiennes's “inner spirituality” onstage, while Jonathan Kent, the director of Oedipus and a longtime collaborator, describes him as “the great actor of his generation”.

Fiennes himself, you suspect, has little estimation of his own talents. His acting, it seems, is itself beyond acting, part of his own delicate process of individuation (remember, “I want to know the secret of my birth”?). All this, a Freudian might add, emerges in the dense undergrowth of childhood, and here it seems particularly poignant and indeed fitting that Fiennes can trace everything back to, yes, his mother, the writer Jennifer Lash.

The Fiennes family history is well-worn lore, and usually involves his farmer-turned-photographer father Mark and mother Jennifer dragging their seven-strong brood (including brother Joseph, sisters Martha and Sophie and foster brother Michael Emery) on a peripatetic childhood whirl through Dorset, Suffolk, West Cork and Salisbury.

In conversation the family tends to describe the period with quasi affection as a time of love without much material comfort. But today, focusing exclusively on Lash, Fiennes is a little more unsparing. He says that she had her own demons, and that, “in her own way, she could make you feel like you hadn't hit the mark, or you simply weren't good enough”.

He elaborates further, describing himself as a child raised on classical literature and recordings of Laurence Olivier's Hamlet, but one driven forward by a desire to impress, coupled by a thundering fear of failure. The childhood ends, and the Fiennes we know begins, sometime around 1982, when he pops up at RADA as an actor in the making. Soon he's on stage, in Twelfth Night, then he's on screen, in Schindler's List via Wuthering Heights.

He ends up here in this room after 15 years in the spotlight, and after one marriage (to the actress Alex Kingston), several relationships (he is currently, allegedly, single, his last reported relationship being a five-month fling with the interior designer Sirin Lewenden) and no children. And this is how we find him - a thoughtful, engaging and provocative actor, contemplating mothers.

We finish with new year's resolutions. He apologises and says that he hasn't any, except to travel more. Where to? “South America, Mexico and Siberia.”

Siberia? “Yes,” he says. “I like it very far north. I went to Nunavut recently, north of the Arctic circle.”

Why? “Because it's very challenging and ...”

Cold? “Yes,” he says, grinning broadly. “Cold, but never soothing.”

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