It's a wrap for rapper Eminem as his screen acting debut impresses the critics.

Film Writer Steve Pratt finds out why he's successful where other pop stars have failed.

Eminem, the bad boy of rap, turned to director Curtis Hanson and said: "Never again" on the last day of shooting his feature film acting debut in 8 Mile. "And he meant it," says the award-winning director of LA Confidential. "It will be a long time before he does another movie where he's in every scene. It was a long and difficult journey."

Talk of Eminem collecting an Oscar nomination as best actor may have been hasty, yet there's little doubt that, if he chose, Eminem could make a living as an actor. Not, perhaps, as the new Cary Grant or new Bruce Willis but, given careful thought, there are dramatic roles the multi-platinum recording artist could handle.

Singers hankering to conquer the cinematic world after climbing the charts and capturing the concert arena are nothing new. What is new is that Eminem, or Marshall Mathers to give him his real name, is so good. He's the exception to the rule that states pop stars make lousy actors. For every Eminen there's a Madonna or Mick Jagger, whose big screen appearances prompt embarrassment more than entertainment.

Eminem succeeded by obeying the essential rules for fledging actors - stick to what you know best until you've learnt how to emote like Kevin Spacey or Marlon Brando.

Although 8 Mile isn't billed as autobiography, the story of an aspiring white rapper from the wrong side of the tracks in Detroit is close to Eminem's background. The world of the city's hip hop clubs is one he knows well.

"I remember, if I lost a battle, it would be like my entire world was crumbling. I would feel like my whole life was over. It's competition. It's like a sport that is somebody's whole life. It may look silly to a lot of people, but to a lot of us, it's our world.

"My character, Jimmy, is really hot-headed, which is how I used to be, and I guess still can be at times."

In Hanson, he had an excellent director, one who not only faced handling an untested actor, but also whose behaviour has earned him a controversial tag.

'I've worked with other actors of reputation, and always found that, if I take them as I find them, they're often quite different than their reputation leads you to believe," explains Hanson.

"I thought that if I thought he could deliver the performance that would work in the movie, then the audience would go with the character and forget about Eminem. I said to myself, 'I don't care about Eminem, I care about Marshall Mathers, the actor who's going to be in this picture'.

"I found him to be thoughtful, serious, extremely focused on his work, quiet, introspective. There was a lot of emotion rolling around in there, a big emotional history. People keep saying it's a biography and I say, 'no, but there's certainly an emotional overlap'.

"He came to the job with respect, trust, and commitment to the story we were trying to tell. Then, the most surprising adjective given his public persona, his humility."

He worked with Eminem on character and script over a six-week rehearsal period, being brutally frank about how hard the challenge would be.

"This was a whole new deal for Marshall. He was very demanding on himself because he wanted to be good. We had our ups and downs, but became close," he says.

"The one thing which gave me the courage to take the leap of faith with him was that it became clear he had that thing, whether you call it charisma or watchability, that makes you interested. When you look into their eyes, you can almost feel you're in touch with their emotions. That's something no director can give to somebody. It's God-given."

If only all acting wannabes from the pop world had Hanson to guide them. Unfortunately, some see doing a movie as a very long trailer for their new album. Back in the 1960s, anyone who had a hit made a movie, spurred on by Elvis Presley's success. After a few attempts at non-singing dramatic roles, the King settled into a cosy groove in 30-odd movies in which he more or less played him and sang a dozen songs.

Britain's answer to Elvis, Cliff Richard, sang his way through a hat-trick of that rarity, successful British musicals, with The Young Ones, Summer Holiday and Wonderful Life.

The Beatles hardly stretched themselves in A Hard Day's Night and Help! but their domination of the charts ensured that there were queues round the block to see them on screen.

Cilla Black, directed by Royal Shakespeare Company boss Peter Hall in Work Is A Four Letter Word, and the Dave Clark Five, in Catch Us If You Can, failed to repeat the Fab Four's screen success.

Some singers never give up. David Bowie was suited to playing an alien in The Man Who Fell To Earth, then landed on his backside with Merry Christmas Mr Lawrence and Just A Gigolo.

After playing Aussie outlaw Ned Kelly, Mick Jagger took the hint and retired from acting to the other side of the camera. He produced the recent Enigma. Elton John also has a film company, following his glittering screen career peaking as the Pinball Wizard in Ken Russell's Tommy. That was based on The Who's rock opera and the group's lead singer Roger Daltrey went dramatic as a real life criminal in McVicar.

Perhaps the Oscar for most persistence should be awarded to Madonna. She's bounced back again and again after being knocked down by the critics and ignored by cinemagoers.

Her cameo as a fencing teaching in the new James Bond movie Die Another Day must have been her reward for writing the title song, it certainly wasn't for her ability to deliver lines.

Even having husband Guy Ritchie to direct her latest movie, Swept Away, failed. Poor business in the US means it's going straight to video over here. She must view with envy the acclaim heaped on Eminem's acting debut.

* 8 Mile (15) is showing in cinemas now.