Theatre seldom works on film, warns Peter Crawley

Of all the pre-show instructions you hear - reassurances about emergency exits, pleas to switch your phone - the announcement forbidding photographs might have a deeper meaning. Flashes and digital shutter-snaps are distracting, certainly, but what they're really getting at is: Don't put this play on film.

Still, there's no telling some people. Opening in cinemas soon, we have a political tête-à-tête between one disgraced former president and one disgraceful TV journalist, in the film version of Peter Morgan's Frost/Nixon. We've been here often enough to know that when film studios turn their attention to successful plays, the results are usually only slightly less controversial than Watergate.

Given that Morgan's play is based on a book about a series of legendary TV interviews, the story has a much-stamped passport between media. But advance reports of "a talkathon embellished with camera movements" sink the heart, as though the gripping intensity of the stage becomes something trapped and inert on the screen.

My hopes aren't much higher for Doubt, John Patrick Shanley's moral detective story, which has snagged two Oscar-chompers on its journey from Broadway to the big screen. Doubt was a meaty enough three-hander and, like Frost/Nixon, concentrated enough to supply two bravura roles. Hence the attraction, one suspects, for Philip Seymour Hoffman and Meryl Streep. If you're inclined to chew the scenery, theatre offers the most roughage.

It doesn't have to be this way. Perfectly supple plays tend to become stale husks on screen because their adapters get restless, expanding the supporting cast, multiplying locations for fear of betraying an airless origin, making the plot leap and judder according to the jump-cut, show- everything philosophy of cinema. But theatre isn't film's anaemic cousin, or its geriatric grandfather.

The best plays turned films - Twelve Angry Men, Glengarry Glen Ross, Ten Things I Hate About You- make confident acts of transposition without just loading up on dolly shots and skipping the interval.

The best example of near harmony between the art forms is how one of the least successful plays in history became one of the world's best films. Never published and unperformed for more than 50 years, it was a wartime drama about a cynical American cafe-owner in Morocco who helps his former lover and her husband flee the Nazis. No theatre producer would touch it. When Warner Brothers bought the rights, they buried the play so deeply that even the female lead never knew it existed.

The play was called Everybody Comes to Rick's, and though it kept the plot, a lot of the dialogue and most of the names, the film settled on a snappier title: Casablanca.

Passing the stage through a lens has been a fitful business ever since - and let's not get started on all those Shakespeares. But it's worth remembering how good the two forms once were to each other. This could have been the beginning of a beautiful friendship.