Watch a clip of Kris Marshall in My Family

Last time I saw Kris Marshall, he was cheerfully burning in hell, sweatily narrating his way through a BBC4 comedy drama called Heist. It was a sort of Men Behaving Badly 700 Years Ago. Call me harsh, but there was something quite satisfying about seeing “mum’s new boyfriend” from those self-consciously modern BT adverts spreadeagled - presumably for all eternity - on a giant toasting rack.

“I’m contracted to do the BT ads for three years,” he says, in a slightly weary tone. “My first chance to opt out is in October. But I’ve learnt to deal with it. It’s not all bad. There’s a hell of a lot worse that actors do.” He glares at me - an enjoyably fierce glare. Then he tells me he chooses a mixture of good scripts (a smallish part in the film Iris, for example) and commercialism. Of course, I say, you have to earn a living. “I don’t have to justify it,” he insists, belatedly.

Then again, Marshall, 35, was barely a jobbing actor for years. He worked as a builder’s mate. He worked on the conveyor belt in a printing works. He lived in Slough. This was not the glamorous existence he had pictured in his teens - admittedly a “rose-tinted view of acting” - when he deliberately flunked his A-levels at Wells Cathedral School, where he had boarded since he was a naive 11-year-old. He enjoyed it, was quite sporty, was never bullied.

He would have become a pilot, like his RAF father, who flew the Queen around the world, but failed the eye test. He had worn glasses since he was five. In any case, he says: “My father will tell you I didn’t have the discipline to be in the forces.”

Marshall had laser surgery three years ago to correct his short sight. Considering he was knocked down by a car at an accident black spot in Bristol three weeks ago, during a stag weekend (“I was merry, but not drunk”), the day we meet, he’s looking great. There’s a cut above his right eyebrow and he’s just had six staples taken out of the right side of his head. “I saw the neurologist last night, and he says there’s a small chance of depression in a couple of weeks’ time,” he says calmly. “But I’m feeling very zen at the moment. He said I’d made a remarkable recovery, considering I did a full cartwheel in the air and landed on my head 12ft away.”

He has no animosity towards the driver (“Just a kid”). He doesn’t remember anything about the accident, which left him in intensive care for 48 hours, but says it has done him good. “I’d got to a point where I was nonplussed about my job. The excitement had tailed off slightly, and this has reset the parameters. But I can’t say it’s changed my life. It was worse for my family and my girlfriend - they were worried, especially when I was in an induced coma.”

Marshall has a short fuse. What sets it off? “People who are stupid, who don’t get it.” Ah. Like those who suggest that the characters he’s best known for are pleasant but slightly bumbling? “Nick was the smartest character in My Family. He pretended to be stupid, but he got away with things.” Something else that annoys him is “people meandering. Make a decision and do something”. He admits he can be forthright to the point of rudeness - “Cutting.”

He shares this trait with Carter, the character he portrays in the American writer Neil LaBute’s new play, Fat Pig, now on in the West End. A typical line: “I don’t think we were put down here to be nice.” Carter is unimpressed by his friend’s attempt at a relationship with a big girl - the “fat pig” of the title. How unPC. “But he is most people’s view on the world.” Some would describe Carter as sadistic. Marshall calls him “fairly arrogant - but a voice of reason”.

“People accuse Neil of misogyny, but this is the opposite. It’s a piece about judgmentalism. There are two strong female characters and two weaker male characters. He touches on people’s flaws without explaining them. He lets the audience think, ‘I’ve done that.’ He makes no excuses for people’s drawbacks.” This play is going to be a big hit with all his friends, he says, because it’s only an hour and 40 minutes long. “With no interval. They’ll all say, ‘Great! See you at the bar at 9.30.’ ” Marshall’s parents split up when he was 14: “Best thing that ever happened. They’d been threatening to since I was five.” (Both are now happily remarried.) He went through a “boisterous” phase - the angry young man. “Thinking I was the dog’s bollocks, when I wasn’t. Not picking fights - as any man who’s been in a fight will tell you, it hurts! But if someone hits me, I hit them back. You have to. It’s the boarding-school way.” He laughs. “I don’t get so angry any more. I try not to give out any shit, but I don’t take any, either. If someone’s in my face, I’ll tell them to eff off.” Ooh, that glare again.

His reddish-brown hair is a mess, in a good way. His accent is soft home counties, his clothes are fashionably casual (Diesel shirt, battered old brown boots). He had an office job briefly, in insurance, and at one point considered going into the City. It’s not hard to imagine him as a maverick trader. “I always took the view that rules were there to be broken,” he says.

But he can be too moody, he says - not depressive, but grumpy.

Marshall’s openness about his own flaws is attractive. “Do I like myself? No! Who does? But sometimes I look in the mirror and think, ‘I’m okay.’ I wouldn’t mind being two inches shorter, though.” He’s 6ft 2in. “It makes you stand out in a crowd.”

Surely he loves the attention? “When I’m acting,” he says. “Not when I’m on a railway platform. I don’t mind being looked at; I don’t like being stared at.” He has the sort of face that can be handsome or plain - which, in theory, means he should get a variety of roles. “People who are dazzlingly beautiful don’t need to be vain; vanity belongs to the ones who can be okay one day and dog ugly the next.” Which category does he fall into? “The latter.”

For seven years, Marshall was broke - he lost his licence for six months for driving uninsured. (“When you’re skint, you cut corners, don’t you?”) Finally, in his mid-twenties, he was cast as Nick Harper, the son in the BBC sitcom My Family, becoming an amiable presence on television for five years until he left the show in 2003. He played another likeable bloke in the film Love, Actually. And now BT Boyfriend: “Someone who’s dealing with having a lot land on his plate.”

So it’s a pleasant surprise that Marshall is not dull at all. He’s fascinated by what can lie beneath a quiet, nice-mannered veneer. “There’s darkness in people,” he says. “At my school, there was a boy who was a genius at the piano, and a meek person, who at the end of the first year was expelled for throwing glass milk bottles from three storeys up at the headmaster’s wife. People like that are the most interesting characters to play.”

His father was “quite stern” and “away all the time”; his mother had an undiagnosed digestive condition that meant she was often in hospital. So, Marshall and his younger sister (now a dental nurse) were often looked after by his grandparents. “In any family with two children, one of each, there’s a certain loneliness,” he says. Really? “My childhood was good,” he says firmly. But then he adds: “In the last few years, I’ve become a lot closer to my parents.”

Marshall has been with his present girlfriend for two years - “a long time, for me. Perhaps I’m not good husband material”. Because he’s unreliable? “No, because I’m a little bit selfish. I like my own company. I wouldn’t rule [marriage] out. But I wouldn’t rule it in either.”

He has lived in Windsor for 14 years and is involved in the local community, to the extent that he recently went round leafleting against the proposed closure of the fire station. “I’m an Anglophile,” he claims, “and I’d miss my friends if I moved to America.”

Marshall says that he’s “on the healthy side of cynical” and sometimes judges people too quickly. But he hates the same trait in others. “There’s no excuse for a complete lack of charm. If you’re going to insult someone, at least do it in a witty way. I can be a little sharp, but I can be all right as well. I can be friendly.”

He can indeed. He’s a softie, really. “I don’t like being criticised, but I’ve had to get used to it,” he says. Which is just as well, because that BT advert is going to run and run.

Fat Pig is previewing at the Trafalgar Studios, SW1

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