Tony Blair. Jesus Christ. Next stop, God?

Of course Michael Sheen wanted to do Jesus. He's already taken on the roles of Caligula, Mozart, Brian Clough and Tony Blair (three times - if ever an actor had reason to complain of typecasting, Sheen has). So I suppose that, having played so many men with Messiah complexes, the next logical step would be to take on the role of the Messiah himself. He even looks a little like the Son of God as he walks into the Covent Garden Hotel, all bearded, with flowing curls and crumpled clothing.

He doesn't turn water into wine during our lunch, but getting him to talk about anything other than his new film, The Gospel of Us, would be a miracle in itself. In a desperate attempt to move the conversation on from the Passion play he put on - and starred in - in his hometown of Port Talbot last year, I tell him I just spotted God himself, Harvey Weinstein, getting into a chauffeur-driven car outside.

"Oh yeah", nods Sheen politely, though clearly not that interested. "He gets everywhere".

Anyway, I really like Michael Sheen. I think he is just about the best British actor of his generation. I was even quite nervous about meeting the 42-year-old, which doesn't usually happen to me. Maybe I had built him up too much in my head, into a deity, a god, even, but by the end of our lunch I feel, if not disappointed exactly, then a tiny bit deflated. He is kind, nice, sparkly, but I get the sense that he is trotting out rehearsed lines, that the film company might as well have stuck a recorded tape of him on the table instead. At least it wouldn't have been in a rush to leave for its next interview.

This national treasure has come over a bit Hollywood. It's not that he is a diva. Far from it, actually. It's more that he has the crafted, well, sheen of a Hollywood star. Perhaps this should not be surprising. He has lived in Los Angeles for years, to be near his 13-year-old daughter, Lily, who lives with her mother, the actress Kate Beckinsale, and has starred in countless movies, from Frost: Nixon to the Twilight saga, not to mention Alice in Wonderland and Tron: Legacy (he is one of the few actors who can do both serious and schlocky and get away with it).

He also had a part in Woody Allen's Midnight in Paris, on the set of which he met the glamorous Canadian actress Rachel McAdams, who is now his girlfriend. But he won't talk about her. When I ask a fairly innocuous question about McAdams and what she thinks of Port Talbot (she accompanied him there to last weekend's Easter premiere of The Gospel of Us), he looks nervously over my shoulder to the two publicists sitting at the table behind us and says: "I don't talk about Rachel. I don't talk about that stuff."

He does, at one point, tell me that he quit Twitter last October because of cruel comments made about this relationship, comments that questioned what a woman like her was doing with a man like him. "And I don't need to read those things. It was just abuse, people writing stupid things, as people do. On the one hand Twitter gives you the opportunity to engage with people, which is great, but on the other there are people who feel they can say whatever they want, put poison out there, really, without fear of any repercussions."

The perils of a socially networked society are touched upon in The Gospel of Us, which condenses the 72-hour Passion play he put on with the National Theatre of Wales into a two-hour film. Sheen co-wrote the production, which took place over Easter weekend last year. It was a huge success. A thousand volunteers from Port Talbot helped with the production, which took place at various locations around the town, from the shopping centre to the beach; 22,000 people went to watch it — most of whom appear in the film brandishing their camera phones as they try to get snaps of Sheen, who played the Christ-like figure.

As he drags his cross through the streets, the most striking thing is not his bloodied body but the number of people trying to take pictures of him. In this way, it is as much a study of the audience as of the actors. "On the morning of Good Friday, when I came down from the mountain to the beach [he slept rough on the mountain to replicate the feeling of being cast into the wilderness for 40 days and nights], the very first thing that happened was someone coming up to me with a camera," says Sheen. "And all I could think was, 'You're f------ it up'. So the first words my character said were, 'Get the f--- out of the way.'?" A wry smile spreads across his face.

"That afternoon, there were thousands of people on the beach, they all had mobile phones and they all wanted to film me. I was freaking out. I thought it wasn't going to work, because all people wanted to do was take pictures of the famous person. I was really annoyed with them. I'd worked on this thing for two and a half years, and everyone was going to miss it because they were too busy trying to take pictures of it!"

In the end he decided to incorporate this into the story, his character asking one man with a camera why he was so desperate to film it. "It became me throwing the money lenders out of the temple. So that relationship is very, very interesting. I mean, the original story [of the Passion] is in one sense a story about fame."

The Gospel of Us is a modern take on the last days of Jesus as told in the Gospel of Mark. In it Sheen plays the teacher, a Christ-like figure who emerges with no memory after 40 days in the wilderness, just as an evil corporation turns up and announces its plans to bulldoze houses and build a new road. It chimed with the residents of Port Talbot, a place often written about for producing such prodigal sons as Richard Burton, Anthony Hopkins and, of course, Sheen. But nobody ever mentions its other claim to "fame", the stretch of the M4 that was erected in the 1960s, cutting straight through the town.

Back then, residents were told it would bring jobs to the area, but they have never materialised. As Sheen's character says in the film, "nobody stops in Port Talbot — they just go through it to travel somewhere else." Sheen may now live in LA but his heart is in South Wales, and he talks passionately about the toxic effect of so-called industry on his hometown. "It has," he tells me, "the highest leukaemia rate in Europe, a high suicide rate and a huge number of carers under the age of 12.

"When people were given the opportunity, through the play, to voice how they felt about all of these issues, about this faceless sort of corporate thing coming in, their feelings were very strong."

Sheen isn't at all religious. He says: "If you can define what God is, I can tell you whether I believe in it." But he thinks that all communities should put on Passion plays — in a strange way, it enabled Port Talbot to create its own God. "Part of what happened that weekend was that people came together and created something that was bigger than themselves. It only worked because of the audience. That's rare, but when you experience it you realise how vital it is, how much you want it. In a way, it's Big Society in action."

I ask if he's ever thought of actually becoming a politician, rather than simply playing one. He smiles. "Not really, no. No, no." That's a shame. His deft deflection of questions shows that he'd most likely make a very good one.

Rachel McAdams adjusts the tie of Michael Sheen at the Sleeping Beauty premiere during the 64th Annual Cannes Film Festival at the Palais des Festivals on May 12, 2011 in Cannes, France