At 20, Lindsay Lohan has become Hollywood's iconic party girl . . . more instantly recognisable in many circles for being tabloid fodder photographed at every A-list event than for her acting.

But Lohan's nightlife finally caught up with her day job last week when a stern warning in the form of a 'shape up or ship out' letter to the young actress from a studio head was widely published across the US.

Lohan was alleged to have repeatedly shown up late on the set of her latest film, Georgia Rule. On 26 July, following a particularly heavy night in which she was snapped stumbling down the street clutching a bunch of roses and singing at a Karaoke bar, she failed to do any work at all. This cost the production hundreds of thousands of dollars.

Lohan had shown up only to be taken to hospital suffering from dehydration and exhaustion shortly afterwards.

Morgan Creek Productions chief executive James Robinson had had enough. In a blistering letter that somehow made its way into the New York Post and various websites, he called the actress "discourteous, irresponsible and unprofessional".

Spoiled Robinson went on to accuse Lohan of behaving like a "spoiled child", adding: "We refuse to accept bogus excuses for your behaviour."

All of this adds up to one thing: Hollywood is very forgiving of its stars' shortcomings and public blunders until it affects their bottom line, which is always the business of making money.

The attitude of the viewing public to any particular incident is much harder to anticipate, since it seems to forgive its stars or crucify them on the roll of a dice.

When Hugh Grant was caught with his pants down in the company of one Divine Brown back in 1995, many thought the man who had built a Hollywood career on a floppy fringe was done for. That 'incident', public relations experts in Britain warned, would haunt him for the rest of his life: he might as well pack up and head back to the hounds on the English country estate from which he appeared to have emerged.

What no one took into account was that American audiences love a bit of unadulterated remorse and a public apology, even if they're not the ones who have been offended.

Grant went on The Tonight Show with Jay Leno, the most-watched late night programme on US television, and addressed the issue head on. He admitted that he had done "a very wrong thing" and said he was truly sorry. Suddenly a man who had been a minor celebrity in the US became a fullblown star, thanks to Divine Brown's oral services and his own apologetic tones.

The key seems to lie in what is and what isn't forgivable. Drug, alcohol or sexual mishaps are perfectly fine . . . once they are acknowledged and time is spent in rehab, and as long as they don't involve children. Robert Downey Jnr, Billy Joel, Colin Farrell, Matthew Perry and a whole slew of other celebrities are testament to that.

Perhaps the award for best recovery, both in image and commercial worth, goes to supermodel Kate Moss less than a year after a newspaper carried a photograph of her snorting cocaine. For her sins, an alleged 150 major advertisers have implored her modelling agency, Storm, to allow them to use Moss in their campaigns. Her popularity and career are at an all-time high and her earnings this year are said to have tripled to over 16m.

Moss didn't even have to offer a major public apology: she simply trotted off to rehab, didn't give interviews . . . she rarely does in any case . . . and continued to look as good as always.

Even bashing people who do not have the same power as you with whatever implement is to hand is pardonable, depending on the star. Think supermodel Naomi Campbell on several occasions and Australian actor Russell Crowe's phone-tossing moves in a New York hotel last year.

Mel Gibson's recent faux pas is in far more dangerous and unpredictable territory, though. Though he has battled alcoholism in the past with little press or public fallout, the demons that escaped from him verbally last weekend may do far greater damage than any physically self-destructive ones.

The spoken word, it seems, can be far more injurious to a career than all the addictions in the world.

A star's choice of words, or which group a star chooses to berate, is even more crucial.

Sexism, in the form of Gibson's allegedly calling a female police officer "sugar tits", he may get away with. Not so talking about "f*****g Jews".

In a Hollywood industry more sympathetic to the Jewish religion than any other faith, perceived anti-Semitism could cost Gibson his blazing career.

It has already cost him a deal he had agreed with US television network ABC to develop Flory, the story of a Dutch Jewish woman and her non-Jewish boyfriend who sheltered her from the Nazis.

For Disney, the studio distributing Gibson's Apocalypto . . . an epic historical drama already hard to market . . . in the US in December, it may be an insurmountable challenge. Several other studios had already opted out of the film because of its violent content and the fact that it's in the Mayan language.

Isolated Gibson may have further isolated a Hollywood he has kept at arms length since his $1bn grossing The Passion of the Christ in 2004. Experts now speculate that he risks losing many in the conservative Christian audience who would have been willing to follow him to Apocalypto.

Though it's impossible to accurately enumerate the effects on a film's profit of a star's fall from grace, certain poor audience turnouts that have followed bad press moments are unlikely to be coincidental. Tom Cruise, for instance, seems to be suffering the effects of two verbal rants in recent times.

One saw him bouncing on the couch while being interviewed by talkshow queen Oprah Winfrey amid gushing declarations of love for Katie Holmes . . . completely reversing the image many of his fans hold of him as the cool, calm and collected superstar.

That earned Cruise unprecedented coverage in every publication and news programme, but didn't win more bums on seats at his films. Though his War of the Worldswent on to make $500m worldwide, many had expected it to take in even more, particularly in the US.

After Cruise compounded his outburst by attacking Brooke Shields for taking anti-depressants (note to celebrities . . . never criticise a new mother unless she's Britney Spears), his expected summer blockbuster, Mission Impossible III, took a weaker than expected $133m.

Though the film banked $400m-plus elsewhere in the world, where the Oprah couch fiasco was not so big a deal, its US box office take was considered very disappointing. As long as Cruise continues to keep his thoughts to himself, his star wattage is likely to shine again, but there are some celebrities who were once enormously successful and seem unable to find any way back to their former glory.

Since his Wham! days in the early 1980s, George Michael has sold a staggering 85 million records around the world. Yet in the US and many other countries he is best remembered not for his music but for an arrest in 1998 for lewd behaviour in Los Angeles, where he propositioned an undercover police officer.

That's not likely to be forgotten in light of his most recent 'incident'. Allegedly caught 'cruising' for a late-night sexual encounter on Hampstead Heath in London two weeks ago, Michael's subsequent lecture to British TV journalist Nina Hossain and threats to sue everyone in sight didn't do him any favours in regaining lost fans.

Blubbering repentance in front of as large a TV audience as possible seems to be the only form of response the public is willing to accept.

Woody Allen, who made no public apology, has struggled to win back American audiences since his affair with and subsequent marriage to SoonYi Previn, adoptive daughter of Allen's long-time partner, Mia Farrow, became public. Allen has made most of his recent films outside the US.