Licence to Kill was Timothy Dalton's second outing as 007. In the film, Bond resigns from the British Secret Service and sets off on a brutal personal vendetta against a master criminal. The 1989 film was initially called Licence Revoked, but this was changed after test screenings revealed that US crowds associated the term with driving. Joe Wright's Pride and Prejudice got a completely different ending in America, including a final kissing scene that was deemed too sentimental for British audiences.

Never underestimate the power of crowds when it comes to the movies. In the final reckoning, it's not the producers, the moneymen, or the stars who call the shots: the reactions of audiences at early test screenings can lead to lines being dropped, scenes being scrapped, even entire plots being rewritten. But where do these screenings take place? How can the musings of a few individuals overrule the will of directors and producers? Just who are these people? And how do you become one?

In fact, there are two types of advance screenings: testers and talkers. They are vastly different, but share one common element - audience feedback. Both allow cinemagoers to voice their views, although the former have the greater effect. Tester screenings act as a dry run for studios to check that their film presses all the right buttons - or not. The target demographic is shown a draft of the movie and distributors then judge whether to make changes. This tactic is well-known in Hollywood; what few people appreciate is that British releases are also tested on home ground.

Working Title's Atonement, for example, was test-screened, with positive results. It might seem a tough one to test due to its literary theme and poignant ending. Not so. The public were pre-conditioned to favour it, says David Livingstone, president of marketing at Working Title, because "they had already warmed up to Keira Knightley and Joe Wright with Pride and Prejudice". Although Atonement was shown to large test audiences, Pride was seen only by executives, families and friends before they decided to axe the ending. So out went the two minutes of Lizzy-Darcy action and in came Donald Sutherland, playing her father, instead. Livingstone also highlights the dual impact of test screenings: their results affect not only the content of the movie, but also how it is marketed.

The thriving website Ain't It Cool News receives and airs verdicts from people who have been to test screenings. These can give rise to a lot of headline-grabbing claims that later prove to have little substance. Recently, there was a rumour that shocked reactions to shots of a body-bagged Heath Ledger in The Dark Knight caused the studio to consider removing them completely. Is there any substance to this? Christopher Nolan's agent says he doesn't test his films - so that would be a no.

And testing is a risk. You have to keep your nerve, especially if you're not willing to make any changes, whatever anyone says. Negative reaction to Se7en was ignored, leaving David Fincher's dark masterpiece intact. On the other hand, Charlie's Angels: Full Throttle went completely unscreened, leading to disaster, at least as far as most critics were concerned.

Given today's increasingly interactive industry, how easy is it to take part in a test screening? The answer seems to be a matter of luck. The process of selection is managed by NRG (National Research Group), the secretive market research company that operates on both sides of the Atlantic. It divides the market into four basic groups, known as "quadrants": males under 25, males over 25, females under 25, and females over 25. (In some cases, the respondents are also divided by race.) From its survey results, NRG projects how well upcoming movies will do against each other in each quadrant, should they open on the same weekend.

Testers, says Livingstone, aim for a core audience with a bit of variety, holding screenings in such places as Fulham and Kensington in London, to High Wycombe and Reading. There, they approach unsuspecting members of the public with clipboards. "Have you seen a film directed by so-and-so?" they ask, singling you out as their ideal test subject. Ten minutes later, free ticket in hand, you can waltz along to a local cinema and then fill in your answers with a brand new pencil. Industry infiltrated.

For those who live further afield, there's the talker screening. A more indirect form of feedback, its purpose is to promote films to the public, to get the word out. Sign up with an agency or newspaper and you can see a film up to a couple of weeks before its release. It's the finished product - you won't prompt any re-cuts - but the studios want responses for future campaigns. It's no coincidence that, at both testers and talkers, the top two boxes on the form, rating the film as "excellent" or "very good", are directly followed by the all-important question: "Would you recommend this film to a friend?"

One company that manages talker screenings is SeeFilmFirst (seefilmfirst.com). As well as creating publicity via word-of-mouth, it also records your opinion. As one researcher says: "Everyone loves a free ticket and in return we ask people to fill in an online feedback form." Whether you're selected for a talker or not will depend on your profile, as a brief questionnaire online determines your type, or types, of movie. If the targeting is off-target, the responses can be damaging. So be quite clear about what genres you love and loathe or you might be responsible for a new - and completely bizarre - cut of Blade Runner.

So remember: clipboards have clout; they are now as essential to film-making as clapperboards. Watch out for the people carrying them. They might just give you the chance to change movie history.