We all know the stories. The pretty shopgirl or the hunky lifeguard is stopped in the street by the talent scout, who says: "You've got that special something! I'm going to make you a star."

Of course, it is not usually like that when Hollywood comes calling for academics. But if the call comes, it is well worth trying to make the best of it.

The culture clashes between academe and the entertainment industry can be pretty dramatic.

When the film The Mummy Returns was launched in 2001, the distributors held a press junket for which the director and stars were assembled and subjected to a barrage of interviews with presenters from countless US television stations. Questions were more or less scripted in advance and thus endlessly repeated. This may not matter to the television channels that serve separate regional audiences, but it can be gruelling for actors, even those who know how to play to the camera and to speak in soundbites. And for academics used to the style of the seminar room it can be something close to a nightmare.

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In this particular case, an Egyptologist was brought in to provide background. Even the scripted questions could be politely described as basic, but one or two of the interviewers felt they had to add a personal touch. And that is how one of the world's leading experts on ancient Egypt came to be asked: "Don't you get frightened working in a room with all those mummies?" And even: "Do you think that a mummy could really come back to life?" He mumbled something along the lines of "That doesn't strike me as frightfully plausible" and vowed to give Hollywood a miss from then on.

This is an extreme example, but it is not unusual for academic experts to be employed on mainstream commercial films, on biographical or historical TV docudramas, in short "making of" films for US television and in the "extras" that accompany the re-release of old films on DVD. Getting involved in any of these can not only be fun, but can also generate income and provide a way for an expert to "reach out" to the public (or publicise a book). However, it is still a good idea to think through the implicit "deal" in advance.

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Most films set in the past aim for a reasonable degree of accuracy because it reassures the audience and helps trigger the actors' imaginations. Academics can obviously play a crucial role here. But there are always going to be trade-offs against other factors, most obviously the budget and the need to tell a gripping story. Director Stephen Frears recalls reading the script of The Queen (2006), which won Helen Mirren an Oscar for best actress, and thinking: "But there's no third act! What we need is a shoot-out in Westminster Abbey!" Local accents or medieval dialogue need to be comprehensible as well as sound authentic. Reconstructed palaces may be smaller than the originals, either for financial reasons or because particular scenes require an atmosphere of intimacy or claustrophobia. And an expensive cinema star may not play ball ("All the extras looked exactly right, but Brad Pitt refused to have a haircut").

Experts employed on film sets can find themselves dealing with directors who are themselves obsessive, have read every available book on the subject and who will argue over every detail. Others treat advisers like comfort blankets, paid well to be available for consultation but almost never consulted. But the real problem comes when scrupulous accuracy is used as a selling point for a film and academics are in effect being asked to give it their imprimatur as part of the marketing campaign.

Historians refused to endorse Ridley Scott's Kingdom of Heaven (2005), which features a crusader called Balian of Ibelin - played by Orlando Bloom, according to one critic, "like a backpacker touring the Middle East in a gap year" - who quite anachronistically loses his faith and commits himself to the cause of religious coexistence. But what should a consultant academic do if a writer decides that the only effective way of dramatising the relationship between Elizabeth I and Mary Queen of Scots is by inventing a meeting between them? This is surely a legitimate technique (and may help to encourage interest in Elizabethan history), but many academics are wary of being associated with films that take such liberties.

In contrast, BBC drama documentaries tend to be far more scholarly than the average Hollywood blockbuster.

Jonathan Phillips, professor of crusading history at Royal Holloway, University of London, worked as the consultant and main talking head on Holy Warriors (2005), about Saladin and Richard the Lionheart. Keen to "communicate with as many people as possible", he was delighted to be able to "put filtered-down versions of the new academic Crusader history out there". He also offers one crucial piece of advice: make sure you get to see and approve the final version of the script, after it has been tinkered with by the foreign co-producers and other interested parties.

&#8220The real problem comes when scrupulous accuracy is used as a selling point for a film and academics are in effect being asked to give it their imprimatur as part of the marketing campaign”

Peter Heather of Worcester College, Oxford (soon to take up a new post as professor of medieval history at King's College London), also makes a sharp distinction between Hollywood and BBC docudramas. He enjoyed Ridley Scott's Gladiator (2000) and even admired its research. "You got a sense of how nasty it was. They felt like flesh and blood individuals and, by drawing on the evidence of gladiator cemeteries, (the film-makers) caught something of the essence of the subject," he says. Still, Heather dismisses the main plot as having nothing to do with reality.

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But when he was employed on the final episode, The Fall of Rome, of the six-part series Ancient Rome (BBC1, 2006), Heather was able to help identify the block of story to use, how broad to make the chronology, and which characters to include.

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"I was then asked to read the script two or three times," he says. "They planned to portray Alaric's domination of the situation by callously murdering the first two senators who came to see him. They wanted it to be popular, to have as much blood and guts as possible, but still to be accurate, so when I pointed out the errors they were corrected."

As the thousands of films available on video are released on DVD, there are often opportunities for academic biographers, historians and film historians to share their knowledge of the characters, the background or the making of the movie. Similar slots are sometimes available on what are known as electronic press kits, the behind-the-scenes publicity films for big features.

Toby Reisz of Feasible Films, who has worked on many such projects, remembers interviewing an authority on the White Star Line in the electronic press kit for James Cameron's Titanic (1997); film historian Sheldon Hall of Sheffield Hallam University for the DVD release of the 1964 film Zulu; and Sergio Velastin of Kingston University, an expert on using surveillance systems to spot suspicious behaviour, for the press kit for Steven Spielberg's Minority Report (2002).

In many such cases, the information an academic is being asked to provide is resolutely basic. When a Cambridge professor discussed the Seven Wonders of the World for a recent project, she had do little more than name each wonder, say what it looked like, where it was and whether it still existed.

In essence, a participating academic "is being asked to bring a sort of gravitas. You cannot overestimate the credibility that an English academic gives an American TV show," Reisz says.

He also offers general guidance for academics who get an unexpected e-mail from Hollywood asking them to take part in such projects. "They need to avoid talking in too sophisticated a style or for too long. What is usually required is not exactly to sell the film but to provide information that makes it look good or well-made or epic in scale.

"Since it's mainly for an American audience, you want them to be enthusiastic. Often (UK academics) are rather dry - and for American TV dry isn't good. Someone who comes over very well was Peter Heather, who seems genuinely to hope that he might enthuse people about Roman history, as he says his life was changed after he visited Pompeii the first time. For an academic to say that on camera is brilliant for a DVD. It makes it seem exciting and accessible," Reisz adds.

For academics willing to play the game and able to put aside a few hours of their time, such work tends to make a pleasant distraction from their day job. It can be useful for publicity or profile raising (though production companies will never allow one of your books to be in shot). And always ask to be paid - Hollywood can certainly afford it.

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