We're in the de Valera suite in the revamped Shelbourne Hotel.

The burly six-foot Dublin actor is looking ruefully at a new watercolour portrait of Dev painted on hand-made paper by the artist John Keating. Gleeson is not just synonymous with Michael Collins for anyone who saw the RTE drama series The Treaty. He's now about to become the personification of another of Dev's great foes in HBO's drama Churchill At War:

His Finest Hour. "I've just finished filming it, " he says. "Poor Dev must be freezing in his grave."

Such is the versatility of Gleeson . . . he's played everyone from the gangster Martin Cahill in John Boorman's The General to MadEye Moody inHarry Potter . . . it's conceivable that he could yet make amends by taking on the persona of Dev himself. Never mind that the physique of the two men couldn't be more different. With the magic of digitally enhanced liveaction cinema . . . as demonstrated in Gleeson's latest movie Beowulf, which renders veteran Ray Winstone in the title role as a handsome young Viking hero with rippling biceps . . . there'll soon be no limit to the roles an actor can take on.

"They reckon they're going to be photo-real within a few years, if not before, " says Gleeson. "They'll be able to make it more or less you.

They will be able to replicate you or change you or place you anywhere once they have all the data, which is a bit god-like. People will be performing from the grave."

But do we want that? "I don't think we're not going to have a choice. It's very scary. There's already quite a move to try and get some agreement that will protect people's facial rights. I think it's going to be quite a bizarre place to go. But it's all part of the sleight of hand that acting has always involved. It's all illusion and entertainment and story-telling. I do think the core of it is about exactly that, about performance and the human dilemmas of people. That's always what's been at the heart of myth."

The process is admirably suited to bringing alive the fantastical monsters and mythic heroes and villains that inhabit Beowulf, the oldest surviving epic poem in the English language. Goya-like giants wrench limbs from victims and bite off their heads. Naked seductresses mutate into ravenous snakes. Fiery dragons swoop out of the sky, wreaking havoc. Standing against them are Beowulf and his loyal lieutenant Wiglaf, played by Gleeson, a voice of reason in the mayhem.

Although Gleeson studied English at UCD in the early 1980s, he'd never actually read Beowulf which is written in difficult Old English. "It was a good chance to go back and look at it. Obviously I gravitated towards Seamus Heaney's gripping translation. You have a Scandinavian legend through Anglo Saxon being interpreted in English by an Irish Paddy. Actually I learned very little from it in terms of my own character. Wiglaf in the actual Beowulf is not the same person as in this. He's a younger man who comes quite late in the poem and becomes king. So when I was talking to director Robert Zemeckis I was clear that I didn't want a dumb buddy scenario. I wasn't really interested in that."

Although Wiglaf is Beowulf 's trusted number one and loves him as a hero, he is his antithesis, conflicted by what he wants the truth to be against what he knows the truth to be. Being a man he too is open to temptation, as Beowulf was. As he stands at the water's edge in the final scene watching his hero's funeral pyre burn, the monster Grendal's mother Angelina Jolie beckons him seductively.

Does he succumb or doesn't he?

Gleeson's expression leaves it tantalising to our imagination.

Gleeson was reluctant at first when Zemeckis approached him.

After playing the vengeful Menelaus in Troy and a ferocious renegade red-haired Crusader in Kingdom of Heaven he was wary of being type-cast as the guy who plays epic character roles. He was wary too of the computer process pioneered by Zemeckis in The Polar Express and Monster House.

"But I have to say although I'm not a techno I found it fascinating.

The whole emphasis is on performance. You dress up in your costume and they photograph all the swirls and turns of that. And then you get decked out in this body suit that's very revealing. You do a complete scene all the way through.

You don't have to stop or re-do any of the shots. It becomes like a black box theatre production really, the whole feel of it."

It's much the way he first experienced drama growing up in Artane. His mother was involved in local theatre, but there was no money for props there. "It was like radio on stage, " he says. "It involved a leap of the imagination.

All you had to do was press the right button and people would go with you anywhere. All of that came into play in Beowulf. It's back to the beginning of theatre. The only thing is they take the tape and make of it what they will in the editing room. You always do that with a movie. It's a leap of trust but particularly with this process, because everything can change.

About 20% of a film's budget normally goes on post-production.

Here it's 80%. Beowulf took just six weeks to shoot, but two years in post-production. The whole thing happens really once you go home."

When Gleeson left St Joseph's School he formed a touring drama group with some friends. Joe Dowling spotted him in The Scatterin' and invited him to audition for the Abbey. He opted instead to become a teacher. At UCD he met Paul Mercier and when they graduated their friendship provided the impetus for Passion Machine, a company based at the SFX community hall that revolutionised Dublin theatre in the late 1980s with its premise that there was as much drama on a northside estate as in any American soap. Gleeson and fellow teacher Roddy Doyle wrote plays that related to the everyday lives of working class audiences.

"That's why I'm optimistic about the whole digital thing, " he says.

"There's this dread of actors being just swept away in some sort of banal generic computer manipulation of what humanity is. And certainly on one level there's going be a load of dross, because it's so easy.

But with proper artists involved and people who are interested in performance the motion capture process could be extraordinary, particularly its human aspect."

Winston Churchill, like Beowulf, is a flawed hero who achieved greatness when his people needed him most. "The savage was coming over the hill and he stood up to him, so you go with that, " says Gleeson. "It was truly his finest hour. It would be dishonest to undermine his achievements and I didn't want to do that. But when I started I had a lot of baggage to get rid of, even in terms of whether I wanted to do it or not. I found it difficult to live with the notion that he was an unbridled hero for obvious reasons, coming as I did from the other side of the table at the Treaty. He was charismatic, there's no question about that. He was an extraordinary force in the world, but hugely flawed in so many ways and could have been just a footnote in history had circumstances not turned out the way they did."

The action in The Finest Hour takes place during a week Churchill and his wife Clementine spent in France after the 1945 election. The result count had been delayed to allow the servicemen to vote and their votes to come back.

"Their marriage is in a bad state.

She said if ever she was going to leave him, that's when it would happen. He was impossible. She was afraid that it would kill him if he wasn't elected but it would also kill him if he was. So from a human story point of view that week was make-or-break time for their relationship and within that context we look back through the war."

On the set people would regularly come up to Gleeson. "They'd say, with great emotion, you must be honoured to play this man, which I wasn't quite sure how to answer, really. Churchill was immensely difficult to live with. He was overbearing and a bully. I could question his imperialism and all that.

He was so many different people at the same time. I always felt that about Collins too. So that's where we went." Gleeson is a non-smoker.

So how did he manage Churchill's cigars? "I was forewarned. I'd gone back on cigarettes after doing King of the Castle, my first play at the Abbey, because my character smoked. I'd been off them for six years and it took me another seven years to get off them again. So when I'm puffing cigars as Churchill I'm not really smoking."