Who really has the final say on what goes into a Hollywood movie? The producer? The director? Well, more often than not, it's you. Or at least, an American member of the public whose very ordinaryness makes them incredibly popular with the film studios.

This is how it works. Teams of leafletters descend on cinema queues, with invitations to advance film screenings. After a rigorous questionnaire - to check that you don't work in the entertainment industry - you're invited to what the studios call a work-print screening.

In March this year, six months before the US release of Ben Stiller's new comedy "Zoolander", Paramount screened the work-print at their studios in Hollywood. This is the studio's chance to gauge public opinion, before it's too late to make any changes.

Disguising myself as Mr Joe Public, it took more than an hour to get through security, passport control (really!), and another questionnaire, including a promise not to blab to anyone in the industry.

Inside, the back two rows were cordoned off for members of the production staff, who would check we were laughing in the right places. One of them welcomed us, and explained that the work-print had no end credits, the opening titles were temporary, much of the colour and sound was unfinished, and some of the effects were incomplete.

Ninety minutes of constant laughter later, we were handed feedback forms. One of the questions demanded: "If you didn't rate the movie excellent, why not?" Well, nothing's perfect!

A glimpse at the answers makes you wonder how useful these questionnaires really are. To the question "What would you tell your friends about it?", the man next to me merely wrote: "Funny."

On this occasion, the producers seemed to have been persuaded that the film needed little more than a polish. Interestingly, the biggest change resulted not from the screening, but the September 11 terrorist attacks - when the World Trade Center twin towers were digitally removed from the film's Manhattan skyline.

"Zoolander" opens in UK cinemas on Friday 30th November 2001.