Only a dialect coach can save an actor from their greatest fear - doing a Dick Van Dyke

First, roll your Rs. Next, replace all of your Ws with Vs. Then, devoice your final consonants, make all dental fricatives alveolar fricatives, and for all vowels, move your tongue closer to your palate. Got it? Congratulations - or, rather, keen-grryea-ch'lyeh-shinz. You now sound a bit like a Russian.

Accents are complicated: when you spell out the bare minimum required to pull one off, as above, that much becomes horrifically clear. But in the cinema, we're fixated on them.

When actors use their own accents to play characters from other countries, as Tom Cruise more or less did when he played Hitler's would-be assassin Claus von Stauffenberg in Valkyrie, people get very upset. When they put on different accents, as Tom Hardy did in the Soviet Russia-set Child 44, people get very upset too.

And if that accent happens to be even slightly off-key - well, good luck getting viewers to talk about anything else. Anne Hathaway's haphazard Leeds burr dominated coverage of the romantic comedy One Day, as did Russell Crowe's Michael Parkinson impersonation in Robin Hood. For film critics, bad accents are low-hanging fruit. But they're too juicy to resist.

Not particularly German: Tom Cruise in Valkyrie, with Carice van Houten Credit: Film Stills

So perhaps it's no wonder the accent-coaching business is booming. For her forthcoming action thriller Triple 9, Kate Winslet used the dialect coach William Conacher to make her character's accent - not just Russian, but Russian-Israeli - as accurate as possible.

To play a bouffant-blonde prima mafiosa called Irina Vlasov, she spent hours drilling pronunciation, saying in a recent interview that she was worried she would sound "like something from a Fast Show sketch". She did the same for her Polish-American character in Steve Jobs, working with a California-based coach, Susan Hegarty, who's been helping her since Titanic. Hegarty's been known to join Winslet on set and tap out the rhythm of her individual lines on her hands in between takes, as she did on The Reader.

Actors are obsessed with getting accents right largely because viewers have become obsessed with them getting them wrong.

That is a relatively recent development: voice coaching has been a part of cinema since the advent of sound, but its original purpose was to make everyone sound alike.

During the silent era, voices themselves had been a fantasy - the stars sounded however you dreamt they did. But after the arrival of sound, Hollywood actors cultivated an accent called American Theatre Standard, which was roughly the US equivalent of Received Pronunciation. It had been taught in American acting schools since the early 1900s, and its rules were laid out by Edith Skinner, a vocal coach who had been taught by a pupil of Henry Sweet, the phonetician who inspired the character of Henry Higgins in George Bernard Shaw's Pygmalion.

Unlike RP, American Theatre Standard had the advantage of not being from anywhere - its elegant mid-Atlantic tones sounded as familiar in Los Angeles as they did in Leicester. It was also tight, bright and melodious: qualities that all worked well with the recording technology of the day, which turned looser elocutions into audio mush. The stars of the great screwball comedies of the Thirties and Forties all speak in American Theatre Standard - and thank goodness they do, otherwise in His Girl Friday, Rosalind Russell and Cary Grant (whose native Bristolian accent was by then long gone) would have sounded like two washing machines on a spin cycle.

'Say aah': Rex Harrison as voice coach Henry Higgins in My Fair Lady, with Audrey Hepburn Credit: Rex Features/Everett/REX Shutterstock

In the right context, American Theatre Standard sounds natural-ish. But in an ordinary conversation, it hits you like a rubber dart in the forehead. In Martin Scorsese's The Aviator, Cate Blanchett plays Katharine Hepburn in American-Theatre-Standard overdrive, and the clash with Leonardo DiCaprio's Houstonian croak as Howard Hughes adds an arch comedy to their scenes.

Some actors made their accents part of their personal brand. People queued to hear Greta Garbo talk, even though Metro-Goldwyn-Mayer executives had previously panicked over how audiences would react to her husky, androgynous growl. (They loved it.) And Errol Flynn sounded like Errol Flynn, whether he was playing Robin Hood or Captain Blood.

During the Second World War, cinema toughened up, which meant American Theatre Standard's days were numbered. Film noir demanded plain talkin' - as did the new, naturalistic styles of cinema from Europe, like Italian neorealism. Younger, ambitious stars like Marlon Brando began experimenting with accents: they weren't always successful, but nobody seemed to mind.

Even in classics of the day, such as Gone with the Wind, the accents are all over the place. The most infamous bad accent in post-war cinema is probably Dick Van Dyke's attempt at cockney in Mary Poppins - but as far as Sixties American audiences knew, that was precisely what cockney sounded like.

The sea change came in the Seventies, when a new generation of film-makers poured into Hollywood's studio system, each ready to tell stories in their own authentic voice. Woody Allen directed in adenoidal Brooklynese, Martin Scorsese the bing-bamboom of Little Italy. The first lady of accents was Meryl Streep, who taught herself cut-glass English without a dialect coach for The French Lieutenant's Woman in 1981 and was duly nominated for the Best Actress Oscar.

The following year she delivered an immaculate Polish twang in Sophie's Choice, and won the award outright. Next came rural Oklahoman in Silkwood, note-perfect Danish in Out of Africa, Irish-American in Ironweed, broad Australian in A Cry in the Dark - and four more Oscar nominations with them. (Not forgetting her Margaret Thatcher in The Iron Lady, for which she won the 2012 Oscar.)

The first lady of accents: Meryl Streep in Out of Africa Credit: Photos 12/Alamy

As much as Robert De Niro's physical transformations for his roles in films like Raging Bull and The Untouchables, Streep's accents made her performances an event. Before long audiences began to expect Streep-level perfection from all actors - and became better at spotting it when they fell short. So in the mid-Eighties, the dialect-coaching industry exploded.

The man who saw it coming was Tim Monich - a student of Edith Skinner. Skinner's life's work was to have all actors talk identically, but Monich helped no one to sound the same twice. He went on to teach Cate Blanchett to sound Russian, Nicole Kidman American, Brad Pitt Northern Irish and Tennesseean.

In response to the growing demand, other agencies sprang up. The biggest, Diane Kemp's Big Timber Management, was founded in 1991. A quarter of a century on, it has 18 coaches on its books and at least four Oscar-winning performances under its belt, including Frances McDormand's sing-song Minnesotan police chief in Fargo.

When you think of Fargo, it's McDormand's voice you hear, just as Tom Hardy's Burton-esque growl is inseparable from The Dark Knight Rises and Scarlett Johansson's refrigerated RP is Under the Skin. Some of the best film accents of all are those you can't resist trying yourself - though mercifully, without having an audience to worry about.

Triple 9 is out on Friday