In his autobiography, Barry Humphries observes that he shares "absolutely nothing" with the character he has played for nearly 60 years, Dame Edna Everage.

I am led to doubt this when I walk into the 79-year-old actor's dressing room at the Milton Keynes Theatre. Urging me to sit by him in front of his make-up mirror, he certainly sounds different, a seasoned tenor as opposed to Edna's arch contralto, and our first few exchanges establish that there is a trace of old fashioned gallantry in his manner, as he affects to outrageously underestimate my age where Dame Edna would deal me a whiplash put-down disguised as a compliment.

Even so, Dame Edna would appreciate his wonderful scarf. It is a deep luxuriant forest green with bright egg-sized orange dots all over it, a scarf that demands to be noticed. When a performer always appears as a character of his own creation and never as himself, the temptation is to imagine that behind the make-up is a shy soul who hates to be noticed, but apparently not.

His characters - the most famous being Edna and the louche, bigoted slob Sir Les Patterson - have served Humphries well, but he has had enough of touring and has decided that his show, Eat Pray Laugh, which comes to Edinburgh this week, will be his last. "It may not be the end of these characters, but I cannot go on living in strange hotels and touring," he says without any trace of regret. "Already after a few days in Milton Keynes … it reminds me of Canberra - roundabout dominated."

He is making concessions to his age, then, but that is not to say that he looks it. If his dark hair isn't its natural colour, then it's a very good dye job, and there are sardonic penetrating eyes in that lived-in face. He is tall and too big for the chair he is sitting on. He looks like Les Patterson's urbane cousin - they don't look similar enough to be twins - and describes himself as being in "very good health" in spite of having had a lifelong aversion to all sports and regarding exercise as "the number one killer".

"I'm much older than most actors and I'm very energetic. I hope tonight will not be the night I collapse on stage," he remarks with disconcerting cheerfulness.

At Melbourne University, pushing against the constraints of a suburban upbringing as the son of a master builder and his wife, Humphries became a Dadaist who revelled in absurdist pranks (one involved filling a pair of boots with custard and calling it "Pus in Boots"). Another stunt involved going out in the car with a friend at night and offering people lifts, only to terrify them half to death by driving at high speed.

His characters still enjoy discomfiting people, but has he become more conservative with age? "In a way I think so because people do who have wild youths," he concedes, but then he sits forward and there's a glint in his eye. "On the other hand, I still seem to be able to shock people. You might be able to hear a gasp tonight." As it transpires later, the gasp comes courtesy of Les Patterson - who else - but the gag cannot be repeated in a family newspaper. Let's just say it revolves around where his doctor's fingers may have been.

No, he hasn't lost his edge.

He is amiable and relaxed, but it doesn't make him any easier to interview. He is not rude - far from it - but he does neglect to answer questions. He'll start answering but duck down a conversational byway like a dog that's caught a scent.

Still, he is more than willing to discuss his characters, though insists there is nothing of him in them.

Edna has often been said by journalists to be based wholly or completely on Humphries' mother, and one can see why. He recalls in his autobiography, My Life as Me, how he would often return home as a teenager with second-hand books only for his mother to flinch at the sight. "You never know where they've been, Barry," she would say. "Remember you're out," she used to say to her children to make them behave in public, a phrase that became the title for one of his shows. However, while he admits he has borrowed a certain "astringency of phrase" from her, he adds that aunts, neighbours and radio programmes for housewives also contributed.

His parents were disappointed when Humphries, having discovered the theatre, dropped out of university after two years, but it was the beginning of his rise as a star.

Edna Everage began life in 1955 as a shy housewife in a one-off sketch Humphries performed while part of a touring theatre company. When he first came to the UK in 1959, he never expected to be able to bring an Australian character to the British stage, but Edna thrived. There were successful tours and by the 1980s, Dame Edna had become a sequined harridan with the best backchat in the business. She had her own lavish TV show, the Dame Edna Experience, supposedly set in her penthouse flat, where guests as famous as Zsa Zsa Gabor and Larry Hagman, at the height of his fame after Dallas, would be dispatched by booby trap on their way to the sofa. On stage, she was accompanied by the glum-faced "Kiwi bridesmaid", Madge Allsop, who dressed in beige and never said a word (in reality, says Humphries, the late actor Emily Perry was entertaining company and could do 25 one-handed press-ups).

After an unsuccessful attempt to break America in 1977, Humphries had a hit Broadway show with Dame Edna in 1999 and there followed a regular stint on Ally McBeal as an eccentric lawyer, taking him to a whole new audience.

Humphries personal life has been hardly less eventful. He has had four wives - he is now married to Lizzie Spender, daughter of the late poet Sir Stephen - has four children and struggled with alcoholism in the heady early days of his career. He wrote in his memoirs that his days and nights "merged incoherently" towards the end of the 1960s as his drinking accelerated, before he swore off alcohol on December 31, 1971.

After that, Edna went from strength to strength. Humphries, made a CBE in 2007, admits she still surprises him - so much so that he is never sure what will come out of her mouth. "I am sometimes a bit like a man operating a puppet," he says. "Edna says something and I think, 'I wish I'd said that.' That's because I know the character so well. Honestly, it is a little bit - to use her own favourite word - 'spooky'."

On the dressing table, nestled amid pots of Mac cosmetics and bowls of fresh berries and bananas, there is a pair of her purple sparkly specs with curlicues at the edge of each lens. These specs are so famous that they formed the centrepiece motif for the closing ceremony of the 2006 Melbourne Commonwealth Games. They have almost transcended the Housewife Gigastar herself, if that were humanly possible, rather like, as Edna might put it, the cross has come to symbolise Jesus Christ.

Humphries notes, however, that it is only in the last 10 years that Australians have stopped grumbling about the way Edna and, in particular, Les, portray Australians. "I have been accused of being unpatriotic because I display an aspect of Australian character that they find embarrassing. Pompous voices have been raised, even in parliament, as to whether I should be allowed to do these characters abroad."

He shrugs. "Australians like to be thought of as very sophisticated whereas they are much more interesting than that."

What lingers is the unexpectedness of having survived so many friends. "Peter Cook and Dudley Moore were friends, and the last people I expected would predecease me. They were in a sense casualties of fame. Spike Milligan was a complicated figure. I have got to the point in my life when a lot of people I know have died or are dying, so I realise that somewhere outside the pearly gates is a queue, shuffling nearer and nearer to the celestial box office."

When Dame Edna finally retires, and Les Patterson kicks the bucket, probably in a brothel in Bangkok, who will carry on the glittering tradition of men portraying women? David Walliams, Matt Lucas and the hugely talented Australian actor-writer Chris Lilley all do so and Humphries has great respect for them.

He likes Little Britain and reserves particular praise for Lilley, the Australian creator of Summer Heights High, in which he plays three characters, a fame-hungry song and dance teacher, a preening self-obsessed schoolgirl and a disaffected Tongan delinquent. "Chris Lilley is a wonderful, original writer and an enormously gifted actor of astonishing bravery and perception," says Humphries.

He also considers Sacha Baron Cohen "a brilliant man" and likes Rob Brydon, Steve Coogan and the pub landlord, Al Murray. However, he is firm in stressing that Edna is not famous for being a man dressed as a woman. "Edna is a woman; it does not rely for its humour on the fact that there's a male actor; Edna is a separate creation."

Agatha Christie famously loved Miss Marple but grew sick of Poirot, so does Humphries have a favourite among his stable of characters? Surprisingly, perhaps, he names the lesser known Sandy Stone. Stone was originally based on a Melbourne commuter Humphries once met and considered to be the epitome of dullness.

Perhaps it is a measure of Humphries' age, but in the show Stone is no longer the joke, but a decent trusting old cove vulnerable to cruel fate. Played as a ghost watching his wife in a nursing home where the staff line the patients up in chairs and ignore them, it marks a dramatic change of pace between the raucous Les Patterson segment and the hilarious Edna part that follows. There are no belly laughs, just pathos; it even brings a lump to the throat.

It is not the only surprise at this stage of Humphries' career: he is to take a turn into theatrical management, having been engaged to create a cabaret event as part of the Adelaide Festival in 2015. It will involve him booking acts. Has he ever done anything like it before? "No," he says, sounding surprised but pleased.

Edinburgh will be only the third stop on the tour and Humphries has not performed in the city since a Royal Variety Performance in 2003. He has affection for the place, not only as an aesthete who appreciates the architecture and second- hand bookshops but because his son Rupert went to Edinburgh University and then got a job at Rockstar Games, going on to be a principal writer on Grand Theft Auto V.

Humphries insists that he is offering a money-back guarantee to anyone who does not "laugh in a major way every three minutes". This sounds like a tall order but when after the interview I join the audience for the second night of Eat Pray Laugh, I realise he means it. Les Patterson opens the show - now a TV chef but struggling with a bout of nasty gastroenteritis which has him running to the loo every 10 minutes - and he is as toe-curlingly unpleasant as ever.

The first two rows are sprayed liberally with saliva. Patterson has always been a misogynist, racist, homophobe, and has not mellowed with age; some people, you suspect, are laughing for the wrong reasons. He is joined by a new character, his priest brother, who has a bag of barley sugars and an electronic tag - you get the picture.

Dame Edna, meanwhile, dominates the second half and her acid tongue is so well directed against the hapless audience, that there are people all around wiping away tears. Her famous ad libs are as quick as snakebites. On stage for two-and-a-half hours, Humphries has tremendous energy and never flags or appears to struggle for his lines. At the end, when Edna's famous gladdies are flying into the audience and she is wishing the audience farewell, you are left wondering if we'll ever see her - or his - likes again. n

Eat Pray Laugh is at the Edinburgh Festival Theatre, Tuesday until Saturday (, and at the King's Theatre, Glasgow, February 11-15, 2014 (