Peter Farrelly is keen to stress that he and his brother have a sensitive side. Sure, they like jokes about bowel movements and getting things painfully stuck in zippers as much as the next guy. But when it came to making Shallow Hal, Peter, the older of the two by a year, knew that they didn’t want to make what he calls “a fat joke movie”.

Shallow Hal comes at the end of a year which has seen Courteney Cox’s revert to her prom-night chubbiness in Friends, Eddie Murphy get back into latex for Nutty Professor 2 and Julia Roberts don a fat suit for America’s Sweethearts. So for Hollywood - a town more obsessed than most with thinness - it has probably been the year of the fat people. But Shallow Hal, says Peter, is different - even if it does feature a 300-pound Gwyneth Paltrow. “Shallow Hal is a movie about inner beauty,” he insists. “The point of the movie is that the shell shouldn’t matter. What’s important is getting to the heart of a person.”

Paltrow herself soon found out that, in real life, it was generally otherwise. Once the fittings for her fat suit were complete, the Oscar-winning actress decided to try it out in the New York hotel where she was staying. “I wanted to see how well the suit worked,” she says, “so I wore it in the hotel bar.” For someone used to making an entry and sure of being looked at in whatever room she entered, the reaction came as a shock. “I realised immediately that no one was making eye contact with me, or would even look in my direction,” she recalls. “No one wanted to connect with me. It was a profound, very sad and startling experience.”

Of course, this isn’t real life; it’s a Farrelly Brothers movie. And while Shallow Hal may not be sad or profound, it’s certainly startling. Its central character is an airhead called Hal Larsen, played by Jack Black, the slightly overweight guy from High Fidelity. Hal is two things when it comes to women: shallow (not to say something of a pig), because he will only consider dating totally gorgeous supermodels; and extremely unsuccessful, because no totally gorgeous supermodel will give him so much as a second glance.

But then self-help guru Tony Robbins (real self-help guru Tony Robbins, playing himself) puts him through a course of hypnosis which lets Jack see the inner beauty behind the shell. It’s a bit like those X-ray spex they used to sell in comic books, with two big differences: it lets you see inner goodness rather than nakedness; and it works. Which is when Hal sets eyes on Rosemary (Paltrow).

Eye of the beholder - where the rest of the world sees a 300-pound woman, Hal (Jack Black) sees Gwyneth Paltrow

When it comes to inner beauty, Rosemary is without equal. She is a genuinely sweet-natured person who works in a hospital for children needing special care. It’s just that very few people actually see the inner beauty because of the… well, the shell. Rosemary is enormous. Chairs collapse beneath her in restaurants. Her mere presence in the stern is enough to make the bows of small boats stick out of the water at 45º. Her panties would look loose on a Sumo wrestler. But Hal sees none of this: he sees the kind of woman he has always wanted to date. He sees, in fact, Gwyneth Paltrow.

And so do we - for most of the time, that is, except when we see Rosemary as other people see her, which is to say, as Gwyneth Paltrow in a fat suit. And that, eventually, is how Hal sees her, too. Because of course the hypnosis wears off and he no longer sees the inner beauty but the outer shell. It’s tough on him - too tough: Hal high-tails it out of there. But not for good. Hal is, at heart, a nice guy; this is a comedy; and there are lessons to be learned about life. The fact that Rosemary turns out to be Hal’s boss’ daughter is not really one of them.