Jeremy Irons will always be embedded in the nation's imagination as an elegant romantic. But to understand his approach to acting it helps to look at the buzz he gets from riding monster BMW motorbikes. Since 1999 Irons has been a member of the Guggenheim Motorcycle Club, which rides in pursuit of art to locations that include Lisbon, St Petersburg, Novograd and Las Vegas. Members include Thomas Krens, the director of the Guggenheim foundation; Frank Gehry, the architect, and Dennis Hopper, the original easy rider.

Describing a journey to the Guggenheim in Bilbao, Irons declares: "By the time you get there you're tingling, absolutely in the right condition to look at art. Your skinhas been shredded off and your nerves are exposed."

Irons's eyes are glistening as he describes the scene. We are sitting in the canteen at Sadler's Wells in Islington, London, on a cold, dark evening. For the first time he seems to have lost some of the severity with which he emerged earlier from the rehearsal room.

Delayed by long discussions with his director, his first greeting - before having his photographs taken - is perfunctory, almost abrupt, giving me just enough time to gain an impression of impatient eyes, a beard that would not look out of place on a medieval icon and a tall angular frame.

The next moment he is gone, taking giant seven-league strides across the canteen, so he can make his way upstairs to meet the photographer. When I eventually catch up he is posing for his first photo but gives me a swift look as if to say "Small talk is not welcome".

Once the photo session is over, we have only an hour until he is due to ride off on his noisy bike for a night at the Old Vic theatre, so he starts taking his enormous strides again, this time in the direction of The Fish Shop, which has been set up as the location for our interview. As we arrive, we are ushered downstairs, where he orders a Bloody Mary and an ash-tray.

He is addicted to roll-ups. As we start talking, he takes out the last third of a cigarette he has been smoking earlier and starts looking around fretfully for a light. I suggest that he uses one of the candles on the windowsill behind him and he replies: "No, it's a short cigarette and I'll probably burn my moustache." Alarmed about the threatened conflagration, I look around but no light is forthcoming - so he uses the candle anyway. Luckily, rather than his moustache, it is Irons himself who starts to warm up.

The initial brusqueness, it quickly becomes clear, is because he is still immersed in thinking about the discussions he has been having with his director, Michael Blakemore,about the extremely challenging role he is about to take up in London's West End.

He will have to age more than 15 years. "And, though I feel 74 tonight," he jokes, "I initially thought there were a lot of good actors who were nearer the age." He will also have to sustain a long monologue. "There are a lot of words," he declares with dry understatement. After 17 years away from the West End, he has decided to make his return with "a new playthat caught me because of what it said - I didn't want to do a classic ora remount". That new play is Christopher Hampton's adaptation of the Hungarian book Embers, a highly coded novel about two men at the end of their lives who have stayed apart for decades because of the betrayal by one of the other.

Embers, which was first published in 1942, has an extraordinary history: its author, Sandor Marai, was persecuted by both the Nazis and the Communists and the book disappeared a fewyears after publication, remaining undiscovered until the beginning of this century. It received enthusiastic reviews when it was published again in 2002 but, although I think the world of suppressed emotion that it depicts is fascinating, I have found it somewhat difficult to get into its richly poetic yet occasionally portentous style. I express these reservations to Irons. He nods with a somewhat donnish solemnity and replies: "I'm interested in what touched you about the book - you talk about historical motives but that's not about the gut: did that touch you, and what was it?"

When I reply that it's about the unspoken in friendship, he agrees and then asserts he's at a point in rehearsals where "I can't really [tell you my conclusions about the book] because it's like when you're making a soup. The ingredients are in there, you sort of know how it's going to taste, it's all very fluid, but I'm still on the journey of finding out what it's about."

Irons's vocabulary is filled with these images of taste, touch and feeling, whether he's talking about what makes him pick a new project, the way he acts, or why, for instance, he suddenly broke off his career in the late 1990s to restore a ruined 16th-century castle in Ireland. Throughout the interview, he reveals several contrasting aspects of his personality: Irons the reflective don, Irons the musician ("I play guitar, piano, clarinet, violin, harmonica, none of them terribly well"), Ironswho started out as a frustratedsocial worker, Irons the father oftwo grown-up sons, Irons the gypsy.It becomes clear, however, that throughout his life and career there has been a drive to experience that sensation he describes when he's riding his motorbike: that sense that the outer layer of skin has been removed, and he is experiencing life in all its vibrancy and rawness.

In an earlier interview he has confessed that in film roles especially he likes to feel unprepared and, because he strikes me as an extremely meticulous actor, I ask him exactly what he means by the word "unprepared". He replies by asserting that - like a tennis player - he is always physically and mentally ready to work but that "I don't know what ball's going to come, so I don't know what my response is going to be".

To illustrate, he describes a moment in the 1984 opening night of Tom Stoppard's The Real Thing in Boston when he was entirely unconscious of the gesture he made when Annie, his on-stage lover, announced she was going to leave him.

"I had a letter from someone who'd been at the first night and she said 'How did you know to do that? That was the gesture my husband did when I told him I was leaving him.' I of course then tried to do it again. It was something like this (he raises his hands above his head) and it just felt ridiculous. I never did it again. I lost it. It had gained too much importance when I tried to recreate it."

It's a quarter of a century now since the Sherborne-educated Irons first came to public attention through his role as Charles Ryder in a television adaptation of Brideshead Revisited, and although this established him as a romantic leading man for the 1980s and early 1990s, there is no sense in which he allowed himself to be straitjacketed as "thinking-woman's crumpet".

His Oscar-winning performance as Claus Von Bülow in the 1990 Reversal of Fortune was a successful risk, as was the 1988 Dead Ringers, and though in latter years there have been flops, such as Dungeons and Dragons and The Time Machine, it is striking that he has done everything from Hong Kong intrigue to Disney voice-overs.

He is one of the few actors to win the triple crown of Oscar, Emmy, and Tony but he also admits that, while he has never actually struggled, there was a period in the 1990s when "I was getting very bored and dissatisfied with the work I was doing in film, and I thought I wasn't being very good. I was quite edgy to work with. So I thought 'I must do something that really frightens me and challenges me. And maybe it will help me through that moment of change from being the leading man to being the father of the leading man.'"

So, at the age of 48, he decided to rescue a medieval castle on the coast of Ireland. On the day he saw the ruin "Sinead [Cusack, his actress wife] was wearing a great long cape, and it was flying in the wind, and her hair was flying - and I thought 'she's a wild woman, that's the place for her.'"

His eyes are gleaming again: "I became the general contractor and hired the workmen. I bought the scaffolding and the crane and everything we needed. It was a huge relief when we finished it. It's a lovely example of risk paying off."

And with that he takes his leave, before disappearing into the London night to brave the traffic on his beloved motorbike.