After lunch with the actor Stephen Mangan, who has dashed off for an overdue appointment with his wife, the waiter sidles up with the bill and a query: "Excuse me, but who was that you were sitting with?" The requested information duly delivered ("you might have seen him in Green Wing", I add), the waiter nods thoughtfully and says: "But he seems really nice."
This note of surprise seems common to all the interviews with Mangan, and although forewarned of his unexpected pleasantness I can't quite rid myself of the suspicion that all the arrogant, selfish, insensitive rotters he has played, from anaesthetist Guy Secretan in Green Wing to his egotistical comedian in the 2005 film Festival and his obnoxious tennis enthusiast in Confetti, aren't somehow based in reality.
"From where do I access this deep well of unpleasantness?" Mangan mused earlier between mouthfuls. "It's probably the thing I most fear becoming. It's almost a way of exorcising it."
In a new BBC2 sitcom, Never Better, he is again playing to type, as Keith Merchant, a recovering alcoholic with a boorish knack for alienating people. And although Mangan says "at least he's doing his best", Keith's a selfish, unsympathetic clown in the modern manner of Larry David in Curb Your Enthusiasm and Steve Coogan as Alan Partridge.
"I don't mind being a hate figure because I know it's not me," says Mangan. "I come from an Irish working-class background but went to a posh school, and any type of pretension was quickly mocked at home. I've always had a keen eye for pretension."
Mangan's parents were from County Mayo in the far west of Ireland. They both left school at 14 and emigrated to England at the age of 17, his father working as a builder and his mother in a pub. By the time Mangan was 11, they had made enough of themselves to be able to send Mangan to a public school, Haileybury in Hertfordshire.
"It was more my decision than theirs," he says. "They were very upset that it was a boarding school, but I quite fancied a bit of freedom. My two younger sisters went to the local state schools because they wanted to."
Did he feel he was separating socially from his family? "Well, it's an incredible change in one generation. My upbringing is so fundamentally different to my parents'. It must be strange to look at your child who not only speaks with a different accent but has a totally different view of the world."
Mangan's world became even more different when he went on to study law at Cambridge. A love of acting which began in primary school was kindled at Haileybury where he immersed himself in theatrical biographies and "dreamt of living in digs in Darlington". But it was at Cambridge that his thespian interests fully ignited. "I did about 23 plays when I was there. I spent most of my time in the theatre or the pub."
He did spend enough time in lectures to come away with a 2:2 degree. And had it not been for a personal tragedy, Mangan might now have been trawling through torts and case histories in whatever legal berth a modest Cambridge law degree might have secured him. Instead, his mother was diagnosed with bowel cancer, and died six months later. "I had just graduated so was able to be with her and look after her," says Mangan, her "ridiculously premature" death persuading him to follow his passion for acting.
"She died at 45 and her mum died at 47, and you can't help but think 'I'm in my early twenties and I might only have another 20 years left to me, what's the best use I can make of it?'."
Accepted by Rada, Mangan set out on a stage career that seemed destined for the RSC. "I thought the best route to being the great actor I wanted to be was to play the great classical parts I spent six years touring the world playing Shakespeare, Molire, Shaw, Goldsmith... But I slowly came to realise that the people you are working with are as important as the parts you play, and that there were lots of interesting people working in film and TV."
It must have quite a leap however from Molire to Adrian Mole, Mangan's breakthrough TV role in 2001, playing Sue Townsend's now grown-up teenage diarist in Adrian Mole: the Cappuccino Years, and from Goldsmith to Green Wing. Channel 4's frantically funny, improvised ensemble comedy set in an NHS hospital has proved a springboard for comic talent, currently emerging everywhere in the TV schedules. Tamsin Greig is negotiating modern romance in the BBC1 sitcom Love Soup, Julian Rhind-Tutt and Michelle Gomez were both in BBC1's recent Oliver Twist, and Mark Heap is the same channel's forthcoming adaptation of Lark Rise to Candleford.
Mangan and Rhind-Tutt, meanwhile, have starred in a series of Barclaycard adverts, formerly a lucrative vehicle for Rowan Atkinson. "I said 'no' about five times and then I said 'yes' and then just before we were due to start shooting I said 'no' again. I just didn't want to do them."
Why not? Surely being asked to star in a Barclaycard advert is a healthy compliment, not to mention paying enough to fund a decent-sized London flat? "Actually, my reluctance was inadvertently quite a good bargaining tool because they kept putting the money up every time I said 'no'," he says. "Anyway, what really persuaded me was looking at the TV and seeing Nicole Kidman curled up on a sofa playing a video game or Penelope Cruz doing mascara adverts, or Morgan Freeman doing Barclaycard adverts, and you think, 'it must be OK...'."
Mangan and Rhind-Tutt owe their fame to Green Wing, of course. Did he feel Green Wing was one of those once-in-a-generation flowerings of comedic talent, like Monty Python or Not the Nine O'Clock News?
"I didn't feel that we were part of a group who were going to move forward and take over the comedy world, no; but there was a tangible sense of being part of something special. Most of us had been in enough rubbish to know when you were in the hands of someone who knew what they were doing."
That somebody was the comedy writer and producer Victoria Pile, whose previous credits include the all-female sketch show Smack the Pony. Mangan is unstinting in his admiration. "I would happily work with her and no one else if she could keep me busy for the next five years," he says. In fact, he was in America earlier this year with Pile, working on a pilot for a "Green Wing-esque" cop comedy that wasn't taken up by the networks.
"I found the experience deeply unsatisfying. There are ranks of executives every step of the way, standing behind the monitor, and they all feel the need to say something. Comedy works best when you have one person in charge."
True to his word, however, he is following Victoria Pile back to Los Angeles later this year to work on another one of her projects. "I'm going back there out of sheer bloody-mindedness. The competitor in me wants to make something work out there just to prove to myself that I could."
The competitor in Mangan isn't immediately apparent over a relaxed lunch at the fag end of the Christmas holidays, which were spent in his Cotswolds bolt-hole with his wife, the actress Louise Delamere (Doc Martin; No Angels), and three-month-old son Harry, "quite possibly the cutest boy that's ever been born".
His domestic bliss is only tempered by the sadness that Harry will never know his paternal grandparents; Mangan's father died two years ago.
"We were filming the last series of Green Wing when dad was ill with a brain tumour and I spent all of my free time with him. It's heart-breaking now that I've got a little boy and he'll never know his grandparents."
Mangan recently took two months off work to "sit on a sofa and just look at my son. I think he's getting my eyebrows. I spend most of my days trying to make him laugh sad but true. The day he doesn't think I'm funny will be a crushing one."
And what of that family medical history? Does Mangan have regular medical check-ups?
"I worked with Geraldine James a few years ago and she has a similar medical history in her family. She put me on to Northwood Park Hospital, which is doing research into families and bowel cancer. I enrolled myself and my sisters into that programme and every three or four years I have a camera shoved up my backside, which was embarrassing to start with but since I played a doctor on telly it's even worse. Patients come and start asking me medical questions."
'Never Better' starts on Thursday 10 January at 10pm on BBC2Reuse content