Michael Sheen found himself going to extraordinary lengths perfecting his portrayal of David Frost in his latest film Frost/Nixon.

Michael Sheen is an expert mimic, an actor who specialises in playing famous names.

For the most part, the people he recreates on film – like Kenneth Williams and Brian Clough – are dead, so there is no chance of them complaining about his portrayal.

It was a little trickier when he took on Tony Blair, twice, in TV’s The Deal and the Oscar-winning The Queen, though he was hardly likely to bump into the Prime Minister in the street.

That’s exactly what happened when Michael encountered the man he is playing in his latest film, Frost/Nixon. And he admits he became Sir David Frost’s secret stalker.

The actor looks and sounds uncannily like the British TV personality, who he plays during a pivotal part of his life – the summer of 1977, when he recorded interviews with the former US president Richard Nixon.

The TV shows, in which Nixon finally admitted his guilt in the Watergate scandal, attracted the largest audience for a news programme in the history of American TV.

Michael says: “I avoided meeting Sir David until I’d started playing him.

“It’s a warts and all portrayal, and he’s such an engaging, nice man. The danger was I might avoid going into certain areas if I’d already formed a relationship with him.

“But when I saw him walking down the street one day, I have to admit I became his stalker!

“I followed him around, watching how he walked, and took photos on my mobile phone.

“That’s why there’s a restraining order out!” he jokes.

Fortunately Sir David approves of his portrayal and calls it “a great honour” that conversations he had 31 years ago have now been retold for the big screen.

The gripping, sharply-written film is based on Peter Morgan’s award-winning stage play.

Director Ron Howard, the former Happy Days actor who has achieved plaudits behind the camera for films like Apollo 13 and A Beautiful Mind, has kept the same two leads. In all, Michael has been playing Frost for two years alongside Frank Langella as Nixon.

So why does he find himself playing real people so often?

“I’d like to say it’s because I’m drawn to iconic characters, but when Ron Howard says ‘I’d like you to play this’, I go ‘OK’,” says Michael. “I’m drawn to people who say ‘I’d like you to do this job’. It’s kind of as simple as that.

“The challenge of playing real people is like a tightrope walk. The more research you do, the more you feel protective towards the person. But we couldn’t make Frost look overly competent, because then the suspense of what’s going to happen in the interview would be lost.

“Inevitably you have to play up certain elements which the real person may find some argument with.

“Frost’s technique was to try to put people at their ease during interviews. It suits him to be underestimated.

“The challenge for me was to portray his superficiality and lightweightness, but to also show there’s more underneath.

“It’s not a particularly pretty picture of him, so it says a lot about the man that he’s been so generous about my performance.”

One might wonder whether Michael got a little bored of playing the same man every day for so long.

But he says: “Working with Frank Langella made it fresh. It always felt real when I was with Frank, never like a repeat although we said the lines more than 400 times.

“And getting a new cast for the film injected a completely fresh energy. All I had to do was try to get a word in edgeways with Oliver Platt!”

The strong cast includes Platt, Kevin Bacon, Matthew Macfadyen, Sam Rockwell and Toby Jones.

They help to add great tension to the film, so audiences are kept on the edge of their seats, even if they already know the outcome.

Michael says: “It’s remarkable how the film keeps people on tenterhooks. Someone who’d seen it told me ‘I suddenly realised I hadn’t breathed for a minute’. Any story that makes people so engaged is a good thing.”

Raised in Port Talbot, Wales, the 39-year-old says he could identify with some aspects of David Frost’s personality.

“We had the same feeling of being a bit of an outsider,” muses the actor, who has a daughter, Lily, with his former girlfriend Kate Beckinsale.

“When I moved from Wales to the big city, I felt like I wasn’t accepted.

“Although Frost went to Cambridge University, he was from a middle-class background. There was a similarity with Nixon, too, the feeling of not being allowed into the club and your background holding you back.

“The irony is that Frost is now the ultimate club member who knows everyone. He introduced me to Gordon Brown at the film’s premiere!”

Mr Sheen is now cleaning up in the movie world.

He is in such demand that he has three new films due out shortly and another two in production, with roles as varied as a werewolf and an East Midlands football manager.

In Underworld: Rise Of The Lycans, he plays the King of the Werewolves, while he is Nottingham Forrest legend Brian Clough in The Damned United, the story of Cloughie’s 44 days at Leeds United.

Then there’s romantic comedy My Last Five Girlfriends with Naomie Harris and Mark Benton, the psychological thriller Unthinkable with Samuel L Jackson and finally Tim Burton’s Alice In Wonderland, in which he plays the Cheshire Cat.

With his recent success, he won’t need much help pulling off the cat’s giant grin.

* Frost/Nixon (15) is released on January 23.