When you ponder the reasons for Hollywood queen Cameron Diaz being cast to voice Princess Fiona in Shrek and Shrek 2, being "down-to-earth" isn't the first thing that springs to mind.

Gorgeous, sure. Highest-paid actress in Hollywood, definitely. The fact that she appears on every list ever published of the sexiest women in the world, maybe.

But forget the willowy Californian actress's feted good looks and unparalleled career success. Director Andrew Adamson explains: "Sure, she's a movie star, model and all those kinds of things, but when you get to know Cameron, she's really just a down-to-earth person."

Cameron's earning a cool $10m dollar fee for her part in what will be one of the summer's biggest blockbusters - a spectacular computer-generated fairytale whose key message is to accept yourself for who you are.

The 31-year-old actress says: "This character is so my character, so I feel very possessive of Fiona."

One of the themes of the film is not to give in to pressure to look a certain way - something Cameron, as a former model, knows all about.

"I think body image is something that's always there for girls. The media is at us from all ends, telling us what sort of image is OK."

Fiona started out in the first Shrek movie as a beautiful princess living with a terrible curse. As the green ogre Shrek's newlywed, her real self emerged - and she was more feisty ogress than yielding princess.

"She didn't accept herself as who she was, but as soon as she got out of that tower and stopped listening to all that, she became the person she truly is," explains Cameron.

"An ogre exists in everyone. All she needs is this man whom she loves, and who accepts her. I think that's a wonderful message."

Flatulence is a key theme in the long-awaited sequel, with Cameron reportedly supplying her own indelicate sound-effects for some of Fiona's more unsociable habits.

"There is absolutely no secret that I'm very gaseous, and I fully buy in to the saying 'It's better out than in'," she confesses. And anyway, after wearing that unique "hair gel" in There's Something About Mary, Cameron's no stranger to gross-out comedy.

When the first Shrek proved a mega-box-office hit in 2001, the clamour for a sequel was immediate. Shrek 2, opening on Friday, July 2, with special previews in Worcester tomorrow and Sunday, is already going down a storm at the American box-office, heralding a new dawn in computer-generated technology.

The three-dimensional animated characters look more life-like than ever, and Shrek's face alone has more than 215 "muscles", all operated by computer.

Fortunately, for a star who's as much in demand as Cameron Diaz, she was able to fit the voice work for Shrek 2 around other acting commitments.

"(Making a film like this) is a strange process because it happens over a number of years," explains Cameron. What's lacking, of course, is the interaction with other actors that happens on a traditional film set.

The fact that a pin-up of Cameron's on-off boyfriend, 23-year-old Justin Timberlake, appears in Princess Fiona's castle bedroom was purely down to the drawn-out nature of the project, according to the Californian.

She says: "It takes a couple of years to make these movies and I think they asked him if they could do that before I even knew him."

Now able to command more than $20m a role in a conventional movie, Cameron will soon appear in Curtis Hanson's comedy drama, In Her Shoes. She plays Maggie, a consummate party girl who clashes head-on with her straight sister Rose, played by Toni Collette.