Cameron Diaz learns to act -- The sultry star of ''The Mask'' is learning the craft and proving she's more than just a pretty face

”It was too dark that time,” says acting coach John Kirby as model Cameron Diaz finishes her lines. ”See if you can enjoy it more.” He’s talking about her line reading, but he could just as well be urging her to revel in her career. When it comes to beating the model-turned-actress jinx, Diaz, 21, has every reason to be happy. An Elite model for five years with almost no acting experience, Diaz auditioned last summer for a two-line part in the new $20 million Jim Carrey film, The Mask, and walked away with the leading female role.

Her self-evaluation is succinct: ”I’m a pretty girl who’s a model who doesn’t suck as an actress.” Maybe that’s good enough. Since the 5’9”, 120- pound Long Beach, Calif., native snagged the Mask role — a nightclub singer with a gangster boyfriend — the buzz on her has gotten louder. She’s been featured on Entertainment Tonight and approached by the Letterman show as a possible guest, and has auditioned for a part in the black comedy Things to Do in Denver When You’re Dead.

But the posing-to-acting transition is not easy, as Diaz’s six-week audition process for The Mask shows. Wary about casting an unknown, New Line Cinema had her read with Carrey 12 times. ”I was ready to have a nervous breakdown,” says Diaz. ”I couldn’t eat, I couldn’t sleep. I got an ulcer.” Getting the job made it worse. ”I thought I was going to throw up my first day. I kept thinking, ‘I’m going to disappoint them and they’re going to fire me.”’

They didn’t, and her performance will make it to the screen when The Mask opens in August. But to ensure that her lucky streak continues, Diaz started one-hour coaching sessions twice a week at Kirby’s Hollywood office to work on such actorly stuff as method and motivation. Kirby — actor Bruno’s brother — has probably seen more scripts than most studio heads, but he manages to look intensely interested as Diaz describes the character she’s playing in an audition scene from Denver: ”She’s a ski instructor living in Vermont, dating the kind of guy she should marry.” Kirby mulls this over, then acts out the scene with her. When it’s over, he reassuringly pats her hand. ”So what are your feelings about this other guy (who shows up)?” he asks. ”Let your feelings evolve.” Kirby has coached other models and believes most aren’t made for acting. ”Some are results-oriented,” he says. ”They only go for the pose — they don’t know what it’s like to really reveal themselves.” After five more run-throughs, Kirby pronounces Diaz’s handle on the character near perfect.

Which is good news, because she’s late for a Denver reading and feeling nervous. Yes, she has an acting coach, an agent, and a publicist, and she takes frequent meetings with producers and casting directors. But she knows she has only a slight edge on thousands of others. ”Everyone is struggling to get in,” she says. ”Modeling and this movie got me in the door.”