How rock steady Eddie keeps the balls in the air

Marsan was due at a secure location to read the script for Spielberg's War Horse before he signed the contract. Such is the secrecy surrounding one of the biggest releases of the movie calendar. Just one hitch: baby was arriving the same day. Marsan phoned a friend – Scotland's Peter Mullan, who had already signed. What's the part like, Marsan asked. It's all right Eddie, said Mullan, you should do it. He did. Such is the refreshingly no-nonsense way working-class movie stars sort out their careers.

Junkhearts is the debut feature of doctor-turned-filmmaker Tinge Krishnan. Casting Marsan was quite the coup for Krishnan, given his previous employers, besides Spielberg, have included Mike Leigh (Vera Drake, Happy Go Lucky), Michael Mann (Miami Vice), Alejandro González Iñárritu (21 Grams) and Martin Scorsese (Gangs Of New York).

Dealing with post traumatic stress disorder and homelessness, much of Junkhearts was shot in east London where Marsan grew up. Though hard hitting, it's a breeze compared to Tyrannosaur, winner of three trophies including best film at the Independent Film Awards last Sunday. Directed by Paddy Considine and also starring Mullan and Olivia Colman, Marsan's scenes are among the most difficult to watch. For that he doesn't apologise.

"It's our job to show it as it really is. If we hadn't, it wouldn't have served the issue properly. Women who have suffered might have said, 'Well, it's nothing to what I've gone through.' My job isn't to worry what the audience think about me, my job is to worry about what they think of the film."

Marsan, 43, takes a practical approach to acting. He's methodical, but defiantly not a method actor. Much of that attitude stems from his background. The son of a lorry driver and a dinner lady, he served an apprenticeship as a printer and could have had a secure, relatively well paid job for years. Instead, he opted for acting.

What did his parents think? "They didn't understand it and they kind of despaired, but they did nothing but support me." It perhaps reassured them that he looked on acting the same way as printing, as a trade, a set of skills to acquire. He even sorted sponsorship through drama school from an East End bookie. After slogging his way through gigs above pubs, he made it into the lower slopes of TV. More than two decades later, here he is, an overnight success.

"I'm a great believer in the work ethic, in serving an apprenticeship and learning how to do something. Because I didn't come from a privileged background I had to believe that things were attainable, and if things were attainable then something could be learned."

As such, he is not a fan of the trend for hiring youngsters who haven't gone to drama school. Far from helping working-class kids break into the business, Marsan believes it could work against them. To his way of thinking, if you believe you might be "discovered" one day working in a cafe, you are far less likely to take charge of your own fate.

"That's like telling working-class kids: don't try to do it yourself, I'll come and find you. There's a kid somewhere in Glasgow who has no money in his pocket, goes to a library and tries to read up [on acting]. Those directors aren't interested in them because it doesn't suit their narrative. Their narrative is, 'Look, I've found this poor kid and I've put him in the spotlight, aren't I great?' That puts my back up."

Acting isn't a social experiment, he says, it's a craft. "Don't get me wrong, those kids will be great for those parts but if [they] are talented you want them to have a long and fulfilling career. To have that they have to overcome so much rejection. One minute they're acclaimed, the next minute rejected; the only way you can persevere through all that is if you learn the technique."

One of those he credits with bucking the trend is Greenock's Martin Compston, who starred in his first film, Sweet Sixteen, when he was 18. The two worked together on the acclaimed British thriller The Disappearance Of Alice Creed. "Martin is a very talented and grounded young man and he loves acting. He always chooses things and tries to create roles for himself; he knows the answer is to work rather than anything else."

There's one other talent Compston has: "He's a good kisser." Ah yes, the kissing scene. Done in one take, or was Compston the joker at it? Marsan laughs. "It was a one-take deal."

Next week, Marsan can be seen reprising his role as Inspector Lestrade in Sherlock Holmes: A Game Of Shadows, directed by Guy Ritchie. As ever, Lestrade feels the lash of Holmesian wit, as delivered by Robert Downey Jr as the Baker Street detective.

"A lot of the first film is really just me and Robert kind of jamming together like two musicians. In the second film there's more of that. He's very funny. Half of it had to be cut because you couldn't put it in the film. It was too controversial. But we just went for it and let Guy sort it out at the end."

After Sherlock it will be Spielberg's War Horse in January. When we speak he is filming Snow White And The Huntsman, with Twilight's Kristen Stewart and Charlize Theron. He plays one of the dwarfs alongside Ray Winstone, Bob Hoskins, Toby Jones and others.

The older he gets, the harder he works, and that suits him fine. "When I first went into it I didn't want to be an 'actor', I wanted to be in EastEnders or something, I wanted to be famous. It was only after I learned about acting and fully appreciated it that I began to realise this is what I want to do. I don't want to be famous, I don't want to be a celebrity; I want to do this and do it well." And he does, going by the calibre of directors who give him work.

"What is remarkable about all of them is that they never lose their inspiration. They have the same enthusiasm, inspiration and discipline of a first-time director. The worst people in this business are those who have had a level of success and they're resentful and cynical about the business and so they think you can just cut corners. People like Scorsese or Iñárritu, Spielberg or Mike Leigh, they're as dedicated as a first-year film student. They've never lost that."

What they see in him he doesn't know. "I haven't got a clue. I turn up, learn the lines, dodge the furniture and do the job." All that, and juggling too.

Junkhearts, Eden Court, Inverness, December 9-12; Glasgow Film Theatre, January 11. Sherlock Holmes: A Game Of Shadows, December 16. War Hourse, January 13