Denis Lawson is sitting in a Kosher restaurant in the heart of London's Jewish community in Golder's Green, wearing a bagel on his face. That's right. A bagel. The Scottish actor is here to talk about his lead role in BBC Scotland's forthcoming pilot The Fabulous Bagel Boys - probably the first ever ''cop-deli show'' - and when the photographer asks him to pose with a half bagel where his lips should be, he obliges without so much as a faintly arched eyebrow.

There are other actors who would not be so willing. Imagine said bagel flying across the room and landing in some disgruntled Jewish princess's chicken schnitzel and you get the

picture. But Lawson, the 53-year-old Crieff-born actor whose name has now officially had grafted onto to it the phrase ''Ewan McGregor's uncle'', is not that type of guy. Many performers are one person in front of the camera and another in the wings, but Lawson has no visible joins. From the minute he steps through the door of Blooms' famous kosher restaurant,

Lawson is on stage.

You're thinking ego-maniac. But it's more like hard graft. The star of ITV dramas The Ambassador and Bob Martin is here to publicise a show and, boy, will he publicise it. Even if it means sitting in front of the gents' toilets having make-up applied or eating bowls of chopped herring and liver (he is a lapsed vegetarian when it comes to Jewish food) with a camera trained on his face. He is poked and prodded while he tries to eat his lunch and never complains. In fact, he makes it look like he's having a ball. The bagel incident leads to one bagel wise-crack after another (''I'm a pro! See me, see bagels'', ''Ach, I'm known for my bagel control'').

It's exhausting watching the room spin around him. And that, it seems, is what being Denis Lawson is all about. In a way he's like an actor from a bygone era where the greasepaint never comes off - he requested a make-up artist for this photo-shoot - and the only way to survive is to keep on smiling. With his elegant dancer's poise, the legacy of years of jazz and tap lessons; his groomed greying hair and never-ending supply of banter, it's an act so honed that it is hard not to imagine there is quite another Denis

Lawson bubbling away underneath.

However, occasionally you see a chink in the make-up. The actor still fondly remembered

for his role in Local Hero and bit-parts in

pre-McGregor Star Wars is endlessly charming and way too much of a gentleman to object to a question. But once or twice during the interview he clams up. When I mention that BBC Scotland seem to be styling him as a mature woman's Hamish MacBeth in his role as Jewish policeman DI Morris Rose, his eyes go blank and he deadpans his response. ''Well I am that,'' he says. And it's clear we are going no further.

Later I ask about Michael Barrymore, the gay entertainer hauled through the press after a post-club party at his house led to the death of a man in his pool. Lawson's response is to protect Barrymore, his co-star in Bob Martin, by quipping: ''Well, you know more than me.'' End of conversation. He is much too nice to tell me it's none of my business, but the mask has slipped.

Trouper that he is though, it's back in seconds. And while maintaining it looks like hard work, he seems to enjoy every minute. But there is a dangerous side to this need to perform. He is the first to admit that he has driven himself to the point of nervous collapse more than once. ''I'm more careful now, but in the past I would always have to give 100% of myself to every single show. If it was a matinee and it wasn't quite full I would just kill myself. And at that time I would be filming during the day and performing at night. I did myself a lot of damage physically and mentally during those periods.''

All of which is in sharp contrast to the famously laid-back Ewan McGregor. The Trainspotting star is known for pitching up to interviews scruffily dressed and saying what comes off the top of his head, while his Uncle Denis gives the impression he would rather die than be interviewed with a hair out of place. And while I am prepared for the fact this proud actor may well clam up when asked to comment on his megastar of a nephew, this is where the mask begins to soften. Nowhere is he less scripted and more warm than when he talks about his

family, and McGregor is no exception.

The pair are very obviously close. They love working together and at McGregor's request Lawson made his directorial debut last year by casting his nephew in the play Little Malcolm and his Struggle Against the Eunuchs. For Lawson the abiding memory is that it was ''a very emotional collaboration''. You get the impression it was in many ways the ultimate expression of a relationship based on careful nurturing on Lawson's part, and admiration on McGregor's.

So did Lawson influence his nephew in his chosen career? ''Well of course I did,'' he replies. ''I used to appear at the house buzzing with what I was doing in London. I think that was enough. He was about ten or 11 when he first said to me (Lawson effects a high-pitched schoolboy voice it's hard to imagine McGregor ever possessed) 'I want to be an actor.' It's all he ever wanted to do.'' What he leaves out until I probe him is the fact groovy Uncle Denis also used to pitch up at the McGregor home spreading hippy vibes with his bare feet and afghan coat. Probably as much of an influence on the impressionable boy from Crieff as the most wacky of showbiz tales.

He has no qualms about discussing McGregor's talent, which is carefully managed by the young actor's mother - and Lawson's sister - Carol McGregor. In fact, he says he remembers the moment he realised his nephew was going to be famous. ''He was playing Orlando in As You Like It in his second year at drama college,'' recalls Lawson. ''And he was extraordinary. He was on a completely different level from everybody else. But I couldn't say at the time because it was too much. I couldn't say to him, 'You know you're going to be a huge star.' I couldn't even say it to my sister. It was just silly. But what

surprised me was how fast it happened. That was fantastic.''

Family seems to be the great stabilising factor in Lawson's life. He glows when he talks about his actress partner of 15 years, Sheila Gish, with whom he shares a house in Chalk Farm. ''We met filming in Wales on something called The Uncertain Feeling, a Kingsley Amis adaptation and yes, it was love at first sight,'' he says grinning. ''And it's been great - it's a wonderful relationship. She's very, very important to me.''

He is equally close to Gish's actress daughters Lou Gish and Kay Curram. All three acted together in Lawson's short film The Base Player. Lawson is also immensely proud of his own

son Jamie, who at 21 has just graduated in anthropology and archaeology from Durham University and plans to be a diplomat. No

acting for Jamie then? ''I offered him a part in my film,'' says Lawson. ''But he turned me down.''

The strong family ties also go a long way to explaining Lawson's history of working himself to exhaustion. ''It's a genetic thing,'' he reckons. ''My parents came out of Glasgow during the Depression and both - particularly my father - had very tough childhoods. They fought their way out of it.'' Lawson, while he clearly has no need to, is still fighting lest the wolf show up at the door of his North London home 60 years on.

If he has calmed down in recent years he says he has taught himself to do so - ''I've just kind of trained myself to go to bed'' - although Jack Black's positive thinking therapy Mindstore comes into the picture somewhere. ''If someone had laid a big spiritual trip on me I would not have gone for it,'' he says. ''But Jack is very direct. It's intensely practical stuff.''

It seems unlikely he'll ever become a man of transcendental leisure though. At 78 years old, his mother, who Lawson describes as ''a dynamo of energy'', is only just retiring from the Crieff watchmaker and jeweller's shop she and her late husband ran for most of their adult lives.

For someone who works so hard, Lawson has never really chased the Hollywood buck. Like his closest actor friends Alan Rickman, Kenneth Branagh and Simon Callow he has spent much of his career on the stage, and while his film CV includes roles in Local Hero and Star Wars, he is just as happy to star in short films by lesser-known directors simply because they interest him. While the bulk of his screen work has been on television, he has avoided becoming identified with one particular character.

All of which could be set to change with his role as DI Morris Rose in young Jewish writer David Solomons' first outing, The Fabulous Bagel Boys. Produced by the makers of Hamish MacBeth, it appears this cop - or cop-deli - drama is being vaunted by BBC Scotland as their ratings answer to Scottish Television

Taggart. All of this makes it quite feasible that Lawson could end up hearing cries of ''Inspector Rose, there's been a murder'' everywhere he goes. So how does he feel about the prospect of being identified with DI Morris Rose?

''I know what you're saying,'' he says, ''and it's something I have fought shy for a very long time. But the idea of doing more of this is appealing because the characters are so strong and the writing is so good. It's very funny, very offbeat. And it's quite different from most cop shows. Nothing much actually happens which is probably a lot more what policing is like. And there's a lot of humour in that.''

Playing a cop is another first for Lawson: ''I never thought I'd be interested in playing a policeman, but the Jewish side of the show appealed to me. I've always been interested in Jewish culture, but never had a chance to dip into it. This gave me an excuse.

''I've always felt a very strong affinity with Jewish people,'' he adds. ''Over the years I find I've become very friendly with certain people I've worked with - actors, producers, whatever - and then two or three years later I discover they're Jewish. I do have a very strong suspicion there is Jewish blood somewhere in my family.''

In many ways, he believes, this has shaped the droll wit that is his signature. As a child he loved to go the movies and the performers who inspired him most were Jerry Lewis, Danny Kaye and The Marx Brothers. ''I didn't realise it at the time,'' he says, ''but it was that dry Jewish sense of humour. I really connected with it.''

He feels pleased now to be part of something that makes a real attempt to reflect Jewish life: ''I think it's an interesting move and quite a brave one to set something in a minority culture in Scotland.

~I did feel a tremendous sense of responsibility to be faithful to the Jewish culture. I don't think any of us went in and did 'Jewish acting'. That would have been patronising. If you meet contemporary Jewish people in Glasgow they don't act differently from anyone else. It was important to represent them properly.''

You get the feeling he means it. It's easy for an actor to feign interest in a subject while they're right in the middle of it. Okay so he's cheerleading for the programme but, believe me, nobody eats that much chopped herring for show.

And that's how it is with Denis Lawson. He's acting his pants off 75% of the time but there is something genuine driving him. And nowhere is that more evident than in his relationship with his nephew. He could play down his younger relative's fame, but he draws attention to it by casting him in his first play. Didn't he worry about failing with the eyes of the world trained on him. ''No,'' he says emphatically. ''I didn't worry at all.'' Of course, I think, he may well have worried at the time, but he's sure as hell not going to tell me if he did. And why should he? After all, the show must go on. n

The Fabulous Bagel Boys is on BBC1, 9pm, Thursday