Yin and Yang: How Billy Connolly calmed down (just don’t mention Piers Morgan!)

Billy Connolly's new film, Quartet, is set in a home for retired musicians and many of the cast are bona fide singers and players of orchestral instruments. At the end, there's an unusual credit sequence in which, beside each character's name, we're shown a photo of an early highlight in the singer's or musician's or actor's career.

It's rather moving, the procession of still photographs from this conservatoire in 1965, or that drama school in 1971 – these earnest-looking young people on the threshold of their careers, bowing a violin, singing a nervous Puccini aria, appearing as Puck or Ariel…

As you watch, you can't help wondering what faded snap will accompany Billy Connolly's credit. Billy in his spot-welder's dungarees? Billy in his pink-satin jumpsuit and platform shoes, wowing the crowd at the King's Theatre, Glasgow?

Connolly has been acting in serious movies for some time – it's 15 years since he shared top billing with Dame Judi Dench in Mrs Brown, playing the gruff Scottish ghillie, and unlikely love object, of the bereaved and reclusive Queen Victoria. But at The Soho Hotel, where we meet, he still emanates an air of disconnectedness, a sense that he doesn't really belong in the thespo demi-monde.

Sitting in his black T-shirt, sporting gap-year beads and black-framed granny glasses, he seems to invite the question: how has a Clydeside welder who started life in a wretchedly poor working-class Scottish family, who spent Saturday nights dancing in Territorial Army mess halls with a burning rolled-up newspaper up his bottom, who shot to fame as a potty-mouthed and scabrous stand-up comedian, wound up consorting with film and stage A-listers on both sides of the Atlantic?

His new film is a quality product, a precision-tooled missile aimed at the grey-pounders who loved The Best Exotic Marigold Hotel. It's directed by no less than Dustin Hoffman (his debut behind the lens) and stars Maggie Smith, Michael Gambon, Tom Courtenay, Pauline Collins and Sheridan Smith. A pretty daunting ensemble for a seasoned actor. Had that bothered him?

"Well, when Dustin reeled off the cast on the phone," says Connolly, "I thought, Oh Jeezus. I was about to turn it down but I said, 'No, behave yourself.' I remembered how working with Judi Dench pulled out the best in you, and I thought Maggie and the rest would do the same. Because when you're working with those guys, you've got no alternative but to act. You can't stand waving your arms around as if you're in Crossroads."

How easily he name-drops Judi, Maggie and Dustin, as if he were Hollywood royalty – but he has earnt himself a seat at the top table in Tinseltown by sheer force of personality. People may ask: Yeah he's funny but can he act? Has he range? Can he do tragedy? (To which the answers are a) Yes, b) Don't know and c) Probably.) But it hardly matters. These days he gets offers right at the top of the acting tree. "About a year or two before that call, Dustin had asked me if I'd like to do Waiting for Godot with him in LA – but it never came off."

Does he worry about not being classically trained, like the Gambons and Courtenays? Do they seem to have secrets he doesn't possess? "The only thing I envy them is their dramatic education. It seems to me dark and secretive, how they go off and prepare their character. I'd like to know more about that – but if I really wanted to know, I'd have done it by now."

The film, adapted from a play by Ronald Harwood, is a chamber piece, weaving the lives and destinies of four ageing singers who once performed a definitive aria from Rigoletto. Three of them (Courtenay, Collins and Connolly) are living out their twilight years in the musicians' home and planning to celebrate Verdi's birthday with a rendition of the aria. Then the fourth shows up. Played by Maggie Smith, she was once briefly married to Tom Courtenay. Can they settle their differences and get together for an anniversary chorus? If the premise is as thin as a Rizla paper, the playing is substantial, nuanced, moving and funny.

Connolly turned 70 last month. When he was offered a film set in an old people's home, was he appalled? "Och no, I've always liked old guys. When I was a boy, there were these old guys who used to play dominoes in the park sheds. We liked them because they had knives and cigarette cards, and they'd tell you stories about the First World War. They called themselves Old Contemptibles, because the Kaiser had called them 'a small, contemptible army'. I thought that was brilliant of them. I always thought I'd like to be one of them when I got old."

Connolly's character Wilf is a bit of a caution – a priapic stroke victim who's wearisomely saucy with the ladies, who smoulders with ageing lust even while gliding on a Stannah Stairlift, and propositions sexy doctor Sheridan Smith. How did he build up the character? "I got the dirty stuff down first – the stroke," says Connolly. "You have to hold your left arm as if you've no control over it."

And the hello-ladies routine? "I found Wilf's lewd stuff kinda one-dimensional and grubby after a while," he says as if discussing a tiresome acquaintance. "I got bored with him. But there was a scene in which Michael Gambon says to Pauline Collins, 'Put your hand up if you're going to speak to me,' and I crossed the room and put her hand down and told her, 'Don't put your hand up for anybody…' After that, I looked after her and watched out to see she was OK. It gave me another dimension, and made me a nicer, more acceptable guy." Wait a minute, I say. Do you mean you improvised a line to change your character and build your part? Didn't the director object?

"Och no, Dustin encourages you to ad-lib," says Connolly sleekly. "He doesn't come with a big preconceived notion of how everything's going to be. He's a great believer in what the actors think it's going to be, so you get to do it first and he tweaks it." Did he give notes to the veteran cast? "What he does, he says, 'Let's do that again so that, this time, you come in from over there.' And you come in from there and the whole scene comes to life, because he knows shit. Being an actor, he knows how to let actors feel they're on the right track."

onnolly can talk in actor-speak but seems happier recalling the old days, in the 1970s, when he first became famous, when he still played folk guitar at music-comedy gigs and when he appeared on Michael Parkinson's chat show and told the joke about the man who murdered his wife and buried her with her rump sticking out of the Tarmac, because "I need somewhere to park mah bike." Mention any famous name to Connolly and he'll weave a story that connects a dozen more. Like this:

"Pauline Collins [who plays Cissy in Quartet] is terrific and great company. I'd never met her before, but I knew her husband, John Alderton. He once beat me at snooker in Scotland. And once we were both prizes in a contest. Schweppes had a contest in which people had to write a limerick about a guest on the Parkinson show who they wanted to meet. There was a big dinner in Park Lane for the winners. A very nice couple from Littleborough won me, and someone else won John, and we were on the same table. I gave him my tie – I'd written on it, 'Where does the yellow go?', because I could never remember the order k of the balls in snooker, and John taught me. They were great occasions to meet people. Henry Cooper was there and Morecambe and Wise, and Barbara Windsor and Diana Dors. I liked Barbara because she was jolly and bright and sparky, but I loved the legendary quality of Diana Dors. She'd just done a serious movie, playing a wicked old woman with a wart on her nose. She told Parkinson that the wart was a stuck-on Rice Krispie, and she had the whole country in stitches…"

Given his easy congeniality with everyone he meets (the latter half of his wife Pamela Stephenson's biography, Billy, is a breathless sequence of Meetings with International Superstars), I was surprised to read, just before our interview, tabloid reports that he'd been in an altercation with Russell Brand on a film set. Reportedly, Brand had repeatedly told a young assistant that he wouldn't film another frame until she'd shown him her breasts; and Connolly had stepped in to tick him off for harassment. Why did he…?

"That [widely reported] story," says Connolly evenly, "is a total invention. A complete fabrication. It's total bollocks. It never happened. Russell was very well-behaved, and I found him very interesting." Did he find him funny? "Oh aye. I really enjoyed his company. I liked his vocabulary, and his stance. He poses and… stances around all the time, and I like that."

You do a great line in vituperation, I say, about all manner of people, from hecklers to suicide bombers. Do you get more furious as you get older? "No, there are very few people I disapprove of. As a matter of fact they're mostly journalists. People like Piers Morgan, who fail upwards." Why does he dislike Morgan? "Because I think he's a talentless wanker, and he goes from strength to strength on the backs of other talentless wankers."

You think he's talentless because he doesn't ask interesting questions? "Because he asks sleazy questions," says Connolly, suddenly very serious. "Because he's a sleazebag." Why do people queue up to be interviewed by him? "Because they're going to be on prime-time TV. I don't understand it. It's like people who go on with the other English guy who did Jimmy Savile – Louis Theroux. There's no shortage of wankers out there."

Inevitably, Connolly met the late, disgraced Savile some years ago. "I met him only the one time at The Savoy. I was doing some charity thing with the Radio One guys – John Peel was a great hero of mine. We were all at a dinner table, and I suddenly felt these hands on my shoulders. I turned round – and there was this face, and the mouth going, 'Now then, now then, uhuhuhuhuhh.' I said, 'Whoa, Jimmy Savile, how're you doing?' I never met him again." You were on Top of the Pops, I say. Do you remember it as a hotbed of paedophilia?

"I was on the show once," counters Connolly, "doing 'D-I-V-O-R-C-E' [his comic pastiche of Tammy Wynette's lugubrious country classic] and 10cc and Slade were on the same night. But I don't remember there being young girls around. I just remember guys on scaffolding, people with cameras. I hardly remember an audience being there, let alone groupies." He sighs. "Certainly nobody exposed me to any culture of shagging."

What did he make of Savile as an entertainer? "He was never funny. But he had his own gig – all that jewellery, clanking and clonking – and that was OK with me."

Child abuse is a sensitive subject with Connolly. As his wife's book revealed, between the ages of 10 and 15 he shared a sofa-bed with his father in the family's cramped living-room. His father, a strict and devout Catholic, regularly "interfered" with Billy – something he kept secret until he told Pamela in a car outside the Glasgow hospital the night his father died.

The link between religion and furtive sex intrigues him. He brings up the Catholic Church's sorry record of abusive priests. "Some people were saying, 'These priests, they should be allowed to marry.' I said, that is an insult to every single man in the world. To say, 'You're single, therefore you're a paedophile.' They're not paedophiles because they're single. I asked my wife what she thought. She said, 'They're not paedophiles because they're priests. They're priests because they're paedophiles.'"

Interestingly, he once played a paedophile priest in an episode of The X-Files. "They said, 'Do you want to meet some of them for research?' I said, No thanks, I've seen plenty of them interviewed and I've never seen a sorry one yet. They all blame it on God – 'He made me the way I am.'" Is he getting more or less religious as he gets older? "I'm totally non-religious," he says firmly. "I've absolutely no idea if God exists. It seems unlikely to me but then – does a trout know that I exist?"

He and Stephenson live in Manhattan these days, but the couple sometimes visit the house they bought in Aberdeenshire. Has he strong views about Scottish devolution? "I don't believe in it. I don't believe in having more layers of government that ordinary, innocent people will have to pay for. I think it's time for people to get together, not split apart. The more people stay together, the happier they'll be."

I ask who, amid the constellation of starry names he has encountered in the past three decades, was he most pleased to have met? "Keith Richards and Bob Dylan," he says without a second's thought. "I've met Dylan six times so far, and he's never known who I am – but I always get a big farewell – 'See ya, Billy!' – from him at the end. But then George Harrison said that sometimes Bob would come to his house and not mix with him at all. He'd go up to a room and play the harmonica."

When Connolly was starting out in the 1970s, comedians on television were vaudevillians such as Ken Dodd, or had risen from the working men's clubs, such as Frank Carson. In this century there has been a massive explosion of stand-up talent who don't do jokes but make wry observations. Is he impressed by the new boys? "I think they're brilliant," he says. "Do you know Tommy Tiernan? I love him." How about his countryman Frankie Boyle, who gets into such hot water? "Frankie Boyle is great," says Connolly. "The thing about people like Frankie is, they let you know where the middle is." Come again? "Frankie's holding up one end of humour and Ken Dodd's holding up the other. So you can make your decision about where exactly you sit, and where your taste is."

It is perhaps a subtle admission, from a man who once held up the extreme end of humour, that there is no harm in maturing into serenity. Not when it brings you transatlantic stardom, the company of the world's leading actors, and Dustin Hoffman whispering "Good job" in your ear.

'Quartet' (12A) opens nationwide on 1 January