And that delighted Diaz, the bubbly star of films like "Charlie's Angels" and "There's Something About Mary."
"In Her Shoes," which also features Toni Collette as Diaz's older sister and Shirley MacLaine as their grandmother, opened at the Toronto Film Festival.
Hanson told reporters that among the film's challenges was getting audiences to accept familiar stars in unfamiliar roles and getting the actors to see themselves that way as well.
"The re's this flawed character who is allowed to cruise through life based on her looks - I thought 'Jeez, Cameron can do that'," he said. "What I wanted to know is, can she go where she needs to go, to be thoroughly despicable?"
It seemed no problem for Diaz, a younger sister in real life, who confessed to being self-centered now and then.
"Their relationship, that Maggie would create chaos constantly and Rose would clean it up constantly, I just thought that was a wonderful story and something everybody can relate to," she said.
"I can be pretty self-centered at times but I don't think to the point where I create chaos, am completely selfish and ruin other people's lives."
Diaz, who pushed for the project to get made, has slowed the frenetic pace she had been on in recent years, taking months off to travel with boyfriend, pop singer Justin Timberlake.
"I worked one film after another for eight years or so," she said. "At a certain point you just go: Where do I live? What do I have? You get tired of trying to make everywhere you go, trying to make it your home. You just want someplace to put your stuff."
With a screenplay written by Susannah Grant of "Erin Brockovich" fame, "In Her Shoes" is the story of two sisters who lose their mother at an early age and have nothing in common but their shoe size.
Diaz's selfish younger sister embarks on a self-destructive, bed-hopping, chaotic life while Collette is thrust resentfully into the role of responsible older, motherly sister.
But just don't call it a chick flick, a term everyone linked with the film recoils from.
"Chick flick or chick lit diminishes it and is sort of insulting to women and men," said Hanson, who also directed "8 Mile" and "L.A. Confidential."
"Why does something that has emotion in it have to be the feminine product?"
After a bitter battle, the sisters gradually repair their fractured relationship and reunite with a grandmother they thought had died. As the grandmother, MacLaine embodies a quiet desperation in the role that is unlike her usual, ebullient characters.
"I love playing contained," MacLaine said. "I'm going to pattern the rest of my career after Morgan Freeman."
MacLaine plays another grandmother in the upcoming "Rumor Has It" with Jennifer Aniston and was seen in "Bewitched," also as a granny.
"I'm all the hot, new actresses' grandmother," she said dryly.