We are in the Riverside Studios theatre in London. The Forkbeard Fantasy theatre group is ripping into its pre-Christmas pantomime-ish show, Shooting Shakespeare.

The show asks us to suspend our disbelief: it is 1904, and impresarios are vying to produce silent movie versions of Shakespeare. Desmond Fairybreath, actor manager of the Old Queen Theatre, Dame Theodora Liverwort (who has played young female leads for the past 50 years) and assorted would-be-actors and assistants prepare the first filmic version of The Tempest.

After a few minutes something strange happens: the actors dive into an invisible crack in the white backdrop at the back of the set and their filmed images continue with the plot.

The rest of the show is a tour-de-force, as live actors interact with filmed images. A giant projector gets wheeled about the set at startling speed, spewing out images through smoke; the audience becomes the eye of a camera as characters are framed by suddenly changing filmed backdrops; and in one beautiful sequence Prospero's island is brought to life through animated splashes of coloured paint.

Forkbeard has been making this kind of work for more than 20 years but, according to Tim Britton (who founded the company with his brother Chris), the technique had been invented long before. The Lumière brothers first projected their films in Paris in 1895, to an astonished audience. Magician Georges Méliès was there and promptly adopted projections in his own live events, creating impressive effects.

Since then there have been countless examples, from Gertie the Dinosaur in the 1930s to the images used by Brecht in his stage plays, and recent work by Robert Wilson and V-Tol dance company.

"It's quite frustrating," Tim Britton says. "Often it's a bit like the emperor's new clothes. You'll get some well-known theatre critic writing about a show that has film in it as if it's the first time that anyone had thought of it."

It is not only theatre that has flirted with cinema to produce a hybrid form. That first film showing by the Lumière brothers was accompanied by a piano, and so-called "silent cinema" was never truly silent. There has now been a renaissance in the use of live music to accompany films. It seems we have realised that performed music adds some "hear-and-now" excitement.

Recently, Marek Pytel's Reality Film has developed a series of stunning shows, teaming up live musicians with movies. A 3D version of the 1950s science fiction classic It Came From Outer Space featured underground rock group Pere Ubu, including an untamed theramin (a vintage electronic instrument) whooping and squealing whenever the aliens appeared. Another recent show, Michael Nyman and Chris Kondek's The Commissar Vanishes, created powerful new images to accompany the musical narrative of Nyman's score. These are but two of countless examples - some fabulous, some dire - of a seemingly limitless desire in the contemporary arts to fuse music and visual images. On the one hand it has become hard to avoid the catatonically dull projected backdrops accompanying some rock concerts but on the other one can also marvel at full-blown virtuoso displays of sound and image, such as those by the extraordinary Japanese pop star Cornelius.

Perhaps not surprisingly, attempts at analysing these new media-mixes can go horribly wrong. Theatre critics get stuck in theatre and know little of film, visual theorists forget the importance of the sound and music element of (say) cinema, musical writers get stuck analysing semi- quavers and (unless it is opera) do not treat the visual element seriously. Oddly, there doesn't seem to be a historical account of this field, and few guidelines exist about how to even talk about such work.

Thankfully, one or two recent theorists have thrown some light on the subject. Audio-vision, by the French writer Michel Chion, is a fabulous exploration of how sound, music and image interact; the sum is often greater than the parts. The American writer Rick Altman sees cinema as a live event and has helped to re-introduce the notion of performance - not that of the actors but the performance involved in the very act of projecting images to an audience. This performance element of film is seized upon by groups such as Forkbeard and artists such as David Leister in his Kino Club events.

There's no doubt that the trend to mix films with theatre, and musical performances with visual images, is set to increase. Events to watch out for this year include The Balanescu Quartet performing with images by Klaus Obermaier in a Contemporary Music Network tour next month in London; Reality Film's showing of the famed documentary about black American boxer and cause celebre Jack Johnson, with live music by jazz drummer Jack DeJohnnette, in May.

Meanwhile, Forkbeard Fantasy is working on a bona fide movie idea with Get Carter director Mike Hodges. Later in the year it is set to return with its iconaclastic games with theatre and film, when it revives its morbidly funny The Fall of the House of Usherettes.