Joan Rivers stands on-stage in a concert hall in the wilds of Wisconsin, regaling a packed audience with her trademark brand of edgy humour. Suddenly a man complains in a loud voice about a gag she has made on the subject of deafness. He announces he is walking out. Rivers, visibly shaken, subjects him to a volley of invective as he leaves the auditorium.

This encounter — a scene from an award-winning documentary about Rivers which will have its British premiere at this year’s UK Jewish Film Festival — prompts two questions. Why is a veteran performer so riled by a single heckler that she is prepared to suspend her show in order to engage in a (literally) stand-up row? And why did the then 75-year-old grandmother need to trek halfway across the American continent for a one-nighter in the middle of nowhere?

Rivers, on the line from her home in New York, addresses the second question first. “I need to do it,” she says “This job is my joy and my passion. I can also use the money. I live very well but I support a lot of relatives.”

The film, called Joan Rivers: A Piece of Work, focuses on her almost pathological inability to turn down an offer of work and her anxiety when dates are thin on the ground. “I’ll show you fear,” she says in the film’s opening sequence, brandishing an empty page of her calendar — “that’s fear”.

So why, at an age when even the most active of showbiz performers are happily in semi-retirement does Rivers plough on so relentlessly? There is, she acknowledges, something pathological about it.

“When I turn down work I feel guilty, I feel terrible, I don’t know where the next job is going to come from. I don’t want to be sitting here a year from now saying I should have worked that weekend. In this business every job is your last job.”

Her role-model in her quest to tread the boards into her ’80s and beyond is another Jewish comedian, the legendary George Burns, who performed as a nonagenarian. Rivers would love to be a female, surgically enhanced version of him. “How lucky he was,” she reflects. “Here was a man who was doing what he loved until he died at 100.”

Rivers, now 77, consoles herself that, in some ways it is easier to perform with age. The gradual process of disinhibition (that disregard for social conventions which comes with advancing years and makes your granddad embarrassing) is actually an advantage for a comedian. She explains: “It’s getting easier to go on stage. You find out that the only good thing about ageing is that you don’t give a damn anymore. I think my stage act is the best it’s ever been.”

Hence the verbal assault on the Wisconsin heckler. “I reacted strongly. I was more shocked than he was, but what you have to remember is that there are 3,000 people watching the show and one cannot ruin it for the others. I had to shut him up because I wanted the others to see the show. I’m not here to banter with hecklers particularly as this was not a drunk man who thought he was funny. He was an angry man.”

Rivers is angry too — another essential factor in the success of a comedian, she claims. But what makes her angry? “Every comedian is furious. Age makes me angry — I’m unhappy at not being able to open packages anymore.

I’m angry that libraries have gone. I hate children on planes. I’m very shallow so they tend to be little things. To be honest, I think I was probably angry the day I was born, you know, about diapers or something.”

In her earlier years, the former Joan Molinsky had plenty to be angry about. She fought for years to make her breakthrough while her erstwhile Borscht Belt contemporaries were hitting the big time. The only conceivable explanation for her struggles was the fact that she was a woman and female comedians had not yet been accepted.

But while Rivers might be lauded as a trailblazer for women in comedy, this is just one more accolade that irritates her. “I never saw myself as someone other than a comedian who was trying to get work. I really resent it when they say that stuff.”

While some might see A Piece of Work as the classic example of a “car crash” documentary (elderly comedienne fights a losing battle against age), it is also illuminating. There is a shot of Rivers standing backstage just before the curtain goes up in which, despite more than half a century in the business, she looks petrified. She is also mortified when her autobiographical play, A Work in Progress by a Life in Progress, which had been a hit in two US cities and at the Edinburgh Fringe, is panned in London. She can handle the brickbats as a stand-up but she takes criticism of her acting extremely personally. “Acting is my true love. I would like to have been a serious actor and I plan to in the next life. I’m gonna be Meryl Streep Rivers.”

I suggest that her drive to succeed might come have come from her Jewish background — her parents Beatrice and Meyer, were Russian immigrants — but she thinks being Jewish was probably less important than being the daughter of immigrants.

“Immigrants came to a new country for better lives for them and their children and were willing to work very hard to get it. All that has been totally lost in the USA now. We’re all third or fourth generations and we’re as bad as everyone else. But I still have a little of that second generation in me.”

Her story is the classic one of a girl from an immigrant family made good, but it is also flecked by tragedy.

In 1987, when the Fox TV network attempted to sack Rivers’s husband, Edgar Rosenberg, as producer of The Late Show Starring Joan Rivers, she understandably refused to co-operate and the show was cancelled. Three months later Rosenberg committed suicide, leaving her life and that of her daughter, Melissa, in emotional and financial turmoil. The hungry quest for work partly stems from the fact that Rosenberg left debts, while her lifestyle is, she jokes, “how Marie Antoinette would have lived if she had more money”.

She also confronted her grief head on, in an unorthodox but typically Rivers way. She made a TV movie about her husband’s suicide and the therapy she and her daughter undertook as a result. “By reliving and acting through everything, it all came out in the open. It was terrific. I think this is a technique that should be used more by psychiatrists,” she says.

Rivers is famously open about her troubles — there are few matters she will not speak about. “Of course there are some private things, but I guess there are fewer and fewer as you get older. You just speak your mind — who cares already?”

And there is no time for regret in her hectic schedule — well, perhaps one. “I should have slept with more men —that’s my regret,” she laughs.

‘Joan Rivers: A Piece of Work’ will be screened at the UK Jewish Film Festival on November 13. Details at www.ukjewishfilm festival.org.uk