SOMETIMES, Hollywood's secrets spill out in the most surprising ways. In October, two of the stars on Grey's Anatomy, the hottest hospital drama on American television, had a fight on set.

Isaiah Washington was ready to go with a scene, but some other actors were not. He and Patrick Dempsey . . . who both play surgical residents . . . exchanged words and Washington grabbed Dempsey by the throat. According to reports, Washington said: "I'm not your little faggot like that guy." Or something along those lines.

The remark was probably meant as no more than a rebuke of Dempsey, but the gossip-mongers quickly started asking who exactly the "faggot" might be. Was one of the cast-members gay?

So far, so trivial. But then something unexpected happened. T R Knight, a 33-year-old actor on the show who plays an emphatically heterosexual doctor called George O'Malley, issued a statement to Peoplemagazine, saying: "There have been a few questions about my sexuality, and I'd like to quiet any unnecessary rumours. While I prefer to keep my personal life private, I hope the fact that I'm gay isn't the most interesting part."

In showbusiness, this was little short of a bombshell. Hollywood may fancy itself as politically progressive, but the unwritten rule . . .

unchanged in decades . . . is that no actor admits he is gay.

Especially not a young, good-looking one.

Knight's coming out was not the only surprise. A few weeks later, a website called Canada. com posted a nasty item that started: "Nepotism is alive and well in Hollywood." It went on to suggest that Neil Patrick Harris, currently starring in a sitcom called How I Met Your Mother (playing an inveterate womaniser), had secured a guest role on the show for a fellow actor it referred to as his boyfriend. His publicist issued a statement stating that Harris "is not of that persuasion", only to receive more negative publicity on the web from bloggers accusing both Harris and his publicist of hypocrisy and bad faith. Once again, People magazine got the scoop. "The public eye has always been kind to me and, until recently, I have been able to live a pretty normal life, " Harris said. "Now, it seems there is speculation and interest in my private life. So, rather than ignore those who choose to publish their opinions without actually talking to me, I am happy to dispel any rumours, to say that I am a very content gay man living my life to the fullest, working with wonderful people in the business I love."

It's not that there is anything faintly unusual or shocking about the existence of gay actors, but going public is something that simply is not done. "It's a death sentence for your career, " said Eve Gordon, a (heterosexual) film and television actress. "All my friends who are gay keep it secret. They don't even know where to draw the line socially. . . So coming out is an incredibly brave thing to do."

That hard truth is itself a taboo topic of conversation in 2006. This, after all, was touted as the year of the gay movie, thanks to the success of Brokeback Mountain, Capote and other titles featuring openly gay characters. Hollywood likes to think of itself as a bastion of sexual tolerance. When gay men started suffering with Aids in disproportionate numbers in the '80s, Hollywood rushed to raise money and awareness of the disease and destigmatise its causes.

The Aids crisis, in turn, made it easier to break the taboo of depicting gay characters on screen. Tom Hanks's Oscar-winning turn as a lawyer dying of Aids in Philadelphia (1992) was undoubtedly the turning point. Before that, playing gay was regarded a threat to an actor's career. After that, it became positively desirable.

According to Donna Deitch, who wrote and directed the lesbian romance Desert Hearts in 1986, playing a gay character is now seen on a par with playing an autistic character, or a schizophrenic, or someone with horrible physical deformities.

"That's where the acclaim is, " she said. "Actors want to play disadvantaged people, whatever the disadvantage is. Now, if you are offering a gay part, and it's a good part, people are going to jump at it.

That's what's changed."

The appeal also extends to female gay parts: Hillary Swank won an Oscar for her role as a transgendered teenager in Boys Don't Cry, and Charlize Theron came close to winning one as reallife lesbian serial killer Aileen Wuornos in Monster (2003). The difference, Deitch pointed out sardonically, is that lesbian characters almost always end up either in a bisexual love triangle or dead.

But playing gay and admitting to being gay are completely different. Hollywood still adheres to the mentality that American audiences look to their on-screen idols as outlets for their own romantic fantasies and thus need to think of them as strictly heterosexual. The mentality is not necessarily wrong . . . homophobia is certainly widespread in the American heartland, as evidenced by the slew of recent state ballot initiatives condemning gay marriage . . .

but it does suggest a failure of the imagination. Actors, after all, are professionals who make audiences believe they are something they are not. If a straight actor can play a gay character convincingly, why shouldn't a gay actor play straight?

The fear goes deeper still: actors playing gay parts, like Heath Ledger and Jake Gyllenhaal in Brokeback Mountain, or Greg Kinnear in As Good As It Gets, are all unambiguously heterosexual in reality. They are, as one Hollywood actor put it, "beyond suspicion".

You don't have to believe the persistent . . . and denied . . . rumours concerning Tom Cruise to know that Hollywood's most visible leading man is never going to play a gay character on screen.

There are exceptions to the rule.

An openly gay character actor such as Sir Ian McKellen can work unhindered because of the prestige that comes of being a British stage veteran and because he is not expected to play heterosexual romantic leads.

For the most part, fear continues to rule. One actor, who did not want to be named, told a story of asking after a colleague's boyfriend while in make-up. The colleague froze, visibly upset, and later explained that he didn't want his homosexuality mentioned, for fear that word might reach the show producers and jeopardise his prospects of future work.

The actor heaped blame on Hollywood's elite, many of whom are themselves openly gay but still continue to perpetuate an atmosphere of oppression. "A lot of people are working against gays to shore up their own closet door, " he said. "They say it's all about the market . . . if people won't buy it, there's nothing they can do about it."

T R Knight and Neil Patrick Harris endured months of hounding by the likes of Perez Hilton, a particularly shameless online gossip-monger who makes it his business to "out" as many Hollywood people as possible without regard for privacy or libel laws.

Both Grey's Anatomy and How I Met Your Mother are guaranteed multi-season runs, suggesting the two actors will remain in work for the future, but after those shows fall away, it is anybody's guess what will happen next. Either their careers will fall apart or, just conceivably, another big Hollywood taboo might at last be broken.