As if the appalling movie W.E. was not wound enough, here is news that the Duke and Duchess of Windsor, on miserable occasions, when driven to depths of boredom, actually had sex with each other instead of with members of their own sex. All of a sudden the devastated looks on their haggard faces is explained. Who says this? It is "Scotty Bowers" a name that had me thinking of the Bowers who perished with the great, gloomy Scott 100 years ago in the Antarctic; if only our author had been in that tent. And you almost feel he might have been, to judge by Gore Vidal's endorsement of this book: "I have known Scotty Bowers the better part of a century  Scotty doesn't lie."

Neither does he supply his book with an index, so if you're interested in the secret sex lives of the stars, you are going to have to search through these pages for Spence and Kate, Eddie and Wally, Randy and Cary, Rock, Ty and Noel, and Vivien Leigh, who seems to have been the most enthusiastic and voluble bedmate of them all. Scotty had so noisy a fling with her (it's pp 158-61) that he was certain they must have woken George Cukor in a nearby room. Where's the harm in that? Don't we all know now  those of us who care  that George gave Viv secret coaching after he'd been fired from Gone With the Wind? In which case, his being privy to her orgasmic outcries must have enrouged her Scarlett. The secret lives of the stars exist only to make their screened moments shine with glamour and mystery.

Bowers is still alive (in his late 80s) and I suspect he's a more interesting man than his memoir discloses. He was a marine who saw active service in the Pacific, and then he settled in Los Angeles, first as a gas pump attendant and then as a barman and unofficial trickster at Hollywood parties. He was very good-looking, and flexible, so the "tricking" he speaks of began as servicing men who wanted blow jobs or more  Walter Pidgeon (Mr Miniver!) started him off, which says a good deal about the wartime suspension of marital relations in the film Mrs Miniver. Quite quickly, Scotty became not just a trick but a bringer of tricks  an intermediary, a connection  as well as a participant. A good deal of this was done for closeted homosexuals at a time when the gay element in Hollywood lived with the threat and the thrill of exposure, while working in a medium that encouraged all of us to fall in love with male and female role models.

Not many of the names outed in this book have not been outed before, and now that they are all dead it is hard for the ordinary stargazer to know what to believe or trust. The true secret lives of the stars were the imagined existences we gave to them. And so in a business dedicated to storytelling, the possibility of fiction easily eclipses the vulnerability of fact. We'll never know, but that is the threshold of exquisite wondering.

There's only one test case that still stirs up controversy, and it concerns the legend of Kate and Spence. In 2006, William Mann published Kate: The Woman Who Was Hepburn, which is the best book written on Hepburn and her dream of herself. Mann had certainly talked to Bowers and his book claimed that "Kate and Spence" was a publicist's fabrication for the most part, albeit one that involved great affection and an on-screen partnership (nine films) that rescued their careers and which was to the glory and benefit of Metro-Goldwyn-Mayer, a studio skilled at public relations. Whereas, in fact, said Mann, Tracy and Hepburn both had gay sides to their lives.

Many fans of the legend were upset by this. Just last year, James Curtis published an immense and eminently researched life of Spencer Tracy that included an author's note on "the biographies of Katharine Hepburn" which said: "Bowers is full of glib stories and revelations, all cheerfully unverifiable." Curtis went on to say: "There is no hint of homosexual activity in the Tracy papers nor in anything I have seen or learned elsewhere during the course of researching this book."

That is the fruit of six years' serious work, and not to be taken lightly. At the same time, I believe Hepburn did have an active gay life, such as Bowers alleges. For the moment, this is a topic, but soon it will fade away. In that future age it will be easier to see that movie stars and celebrities were and are in the business of mystifying fact and generating stories about themselves. That is a trick we are all learning that is gradually undermining the pursuit of  or the hope for  history.

David Thomson is the author of Have You Seen? (Penguin) and The Moment of Psycho (Faber).

In his greatly underrated 2006 biography of Nicole Kidman, the film critic David Thomson explained why he had made the surprising decision not to meet the actress in the course of his research. For the magic of cinema to work, he wrote, "the actress and the spectator must remain strangers". While honest about the hopeless desire he felt for Kidman while watching her films, he had no wish to encounter her in real life. Thomson places himself among those cinema-lovers who "would always protect and preserve desire by ensuring that it is never satisfied".

Such scruples don't seem to trouble Scotty Bowers. For decades, Bowers  now 88  was the "go-to guy" in Hollywood for "whatever people desired". Bowers was a bisexual gas-pump jockey and later a bartender, a handsome ex-marine who made it his mission to satisfy the desires of the stars as well as his own. From the Richfield gas station on Hollywood Boulevard, he organised "tricks", both gay and straight, for many of the biggest names in town, including George Cukor, Cole Porter, Noël Coward, Errol Flynn, Cary Grant, Charles Laughton and Katharine Hepburn (whom he claims to have fixed up with "over 150 different women"). Where possible, Bowers liked to perform the tricks himself. "I was proud of my dick and I was happy to share it," he writes with characteristic immodesty in this juicy, filthy memoir.

If you share Thomson's love of the screen, I beg you not to read this book. Its endless graphic descriptions of urges being satisfied are not conducive to maintaining the suspended desire you experience in watching the classic films of the 40s and 50s. At one point, Bowers mentions that he prevented the biographer of Tyrone Power from describing "Ty's passion for piss and poop", because it would have shattered the myth of this matinee idol. Yet now he happily shatters the mystique of countless stars, breaking confidences and revealing all manner of embarrassing fetishes  whether for underage girls, smegma or voyeurism. Perhaps he thinks it doesn't matter any more.

He tells of orgies and threesomes, of strange predilections and surprising encounters: what Charles Laughton liked to eat in his sandwiches (you don't want to know), how loudly Vivien Leigh screamed, the oral tendencies of George Cukor. Bowers's world is one of wild parties where, for a little extra fun, he gets out his "swizzle stick" and uses it to stir cocktails. "Folks loved that." He presents his lurid reminiscences as a kind of secret history of the movie business. While the rest of America sought its escapism at the cinema, the stars themselves escaped to the gas station where "whatever folks wanted, I had it".

It's hard to know how many of Bowers's stories to believe. He repeatedly insists that he "never made or wanted a dime out of the tricks" that he arranged, which seems distinctly odd in a place as money-driven as Hollywood. Bowers paints himself as an altruistic figure: "Most of what I did for six decades was to keep people happy." That may be true (though it doesn't take account of the sad story of Bowers's own private life, involving a betrayed wife and a neglected daughter who died aged 23 after a botched abortion). "My operation," Bowers states adamantly, "was not a prostitution ring", though he admits that he was happy to "pocket the tip" when participating himself and does not deny that in the early days at the gas station, he charged people by the dollar to peep on various activities in the toilets. He also admits that he fixed up the silent movie star Harold Lloyd with many "beauties" over the years, all of them "hookers". Yet he still proclaims that he never profited from his role as a fixer.

Maybe  just maybe  it is so. He comes across as desperate to be treated as a friend of the stars rather than someone offering them a paid service. Like Thomson, he watched these "larger-than-life" gods and goddesses in the cinema as a boy. Unlike Thomson, he did everything he could to know them better. Spencer Tracy, Mae West, Laurence Olivier, Vincent Price are all cast as his confidants. He loves it when Cukor bad-mouths Judy Garland for her behaviour on set, because it places him in the role of an insider. He gives his famous clients instant nicknames as a form of intimacy: Spence, Larry, Vinny. Sometimes, though, it all sounds a bit like it's happening in his head.

The most startling examples are "Eddy" and "Wally", aka the Duke and Duchess of Windsor, who Bowers says were introduced to him by mutual friends, including Cecil Beaton. Bowers's line is that Wallis and Edward were both essentially gay and that the reason for Edward abdicating the throne was that their marriage gave him the freedom he needed to satisfy his urges in private. During a visit to Hollywood, Bowers says he arranged all manner of pretty dark-haired girls for the duchess and boys for the duke, as well as offering his own body for the duke to do with what he liked. "Eddy was good. Really good."

"Frankly," he observes, "I cared not one iota whether they were gay, straight, or bi." The most endearing aspect of the book is Bower's extraordinary open-mindedness about the wonderful weirdness and variety of human passions. Nothing fazes him. "When it comes to sex every single person is different, but those differences are usually so slight, so subtle, that unless you're really tuned in you can overlook them or not even be aware they exist." Bowers will frequently describe some outrageous predilection, then go on to say how charming and nice the person was who held it.

Almost the only people he is straightforwardly bitchy about are people he didn't get to service: Rita Hayworth (he accuses her of meanness, adding "we never had the chance to go to bed together") and James Dean ("a very unpleasant young man", whose real offence seems to have been that he had so many "boys and girls chasing after him" that he didn't need Bowers and his tricks). Bowers gives a vivid sense of both the freedoms and restrictions of life as a Hollywood star in the 1940s and 50s: to have the adoration of millions and the money to satisfy your every libidinous whim, without any worries about Aids (which was what eventually put a stop to Bowers and his fixing); yet still to be forced into absurd subterfuge in order to abide by strict moral codes of the studios and avoid the harassment of the vice squad.

It seems sad, too, that these golden idols were driven to satisfy their urges through this one particular guy. However wide their sexual interests, they don't seem to have had much choice in how to meet them. Every star who comes into proximity with Bowers seems strangely shrunken. His impressions are mostly banal. Cary Grant  with whom he shared "sexual mischief"  was "the quintessential Mr Smooth  precise, dapper and debonair". Leigh was tempestuous and highly-strung. Katharine Hepburn was highly intelligent. You don't say. Didn't we know all this  and so much more  already, just from watching them up there, luminous and true, on the big screen? Bowers may have shared bodily fluids and confidences with the stars, but that doesn't give him access to the secret of Hollywood. The real life  the real desire  of cinema is preserved on celluloid for all of us to share.

 Bee Wilson's Sandwich: A Global History is published by Reaktion.