ACTOR Michael Sheen has played real people before. He's been Mozart, Caligula and is preparing to play Dylan Thomas in a new film.

But his role in C4's political drama The Deal was altogether more scary, playing not only a British Prime Minister but one who's very much alive and still in power - Tony Blair.

The Deal deals with Blair's relationship, both political and personal, with Gordon Brown, from their early days as MPs to the meeting in an Islington restaurant in 1994 to determine who would follow John Smith as Labour leader.

The award-winning Welsh actor is not the first person you'd think of to play Blair. Sitting here, talking about the challenges the role presented, he hardly looks like a Tony clone. Friends were of the same opinion when he told them about his new role.

"They'd say, 'that's really strange' and then, 'oh my god, I can see that'," he says. "I don't know what that says about me."

Director Stephen Frears insisted that Sheen was the only actor for the part, and delayed filming until he was free. At one point, he was playing the Labour leader on film by day and notorious Roman emperor Caligula on stage by night. He laughs when asked if he ever muddled up the two characters.

"On the days I was doing both, I was literally going from one set to the other," he says. "In terms of preparing, I was having voice coaching for one and a half months before film. So while I was rehearsing Caligula, I was working on Blair too."

The main problem capturing Blair concerned the fact that he's well-known and still very much in the spotlight. "I've had to learn to do different accents, but not how to speak specifically like someone. It's a fascinating discipline," says Sheen.

"I was playing someone who's so well-known, although our film finished ten years ago. It's a character you feel you know very well, but your references are now rather than when the piece was taking place. Certainly for Blair, he was very different in that period to the way he is now. He was much younger and lighter."

Sheen has never met Blair and wonders how pleased the Prime Minster would be to see him after viewing The Deal. The actor does confess to a strange responsibility towards his subject. "I feel a rather odd affinity to the man and that I should defend him, regardless of his policies or leadership. As a man, you can't help but feel close to him," he confesses.

Before filming, Sheen spoke to many people who knew Blair and to political commentator James Naughtie, who's written about the Blair/Brown relationship.

Blair still remains something of a mystery to Sheen. "In our piece and in life, there's something impenetrable about him. He's difficult to read and keeps up a very affable, charming front. He appears nice and open and genuine, but there has to be another story," he says.

"It was a question of finding a balance. The character is totally non-threatening at first. Then, as time moves on, you start to see something harder."

Sheen and David Morrissey, who plays Gordon Brown, got along fine, although he supposes it would have worked equally well if they'd hated each other. "It's easier to play rivalry and more difficult to be convincing as friends if you're not. The fact that David and I have become very good friends helped a lot to have a real connection with each other," he says.

"We supported each other through it. All the work you've been doing on the character disappears once you play in front of the cameras. You react off each other and that worked really well."

One result of Sheen's research for The Deal was to deter any political aspirations. "It's the banality of it - the day-to-day job would get me down," he explains. "For most politicians, they get into a position to do something and create change, then spend most of their time doing what Blair calls the business of politics."

The Deal is being shown for Labour Party delegates on the eve the annual conference at the weekend, but Sheen doubts that the main real life players will be present. He won't be - he'll be in London at the premiere of Bright Young Things, the Stephen Fry-directed film in which Sheen has a leading role.

He recognises that Blair is the latest in a series of very different parts in movies including The Four Feathers, Heartlands and the forthcoming Laws Of Attraction ("I play a mad rock star, not based on a real person but a mix of Iggy Pop, David Bowie and Lou Reed," he says).

Playing Dylan Thomas represents a bigger challenge than Blair and not just because Sheen will need to put on weight to be convincing physically. "For me, as a Welshman, that will be very daunting because he's still such a huge iconic figure. I want to get him right," he says.

The Deal: Sunday, C4, 9pm.