Cameron Diaz: In a class of her own

Cameron Diaz loves her latest role as a morally dodgy teacher – and the scenes with ex-boyfriend Justin Timberlake were quite painless, she tells Lesley O'Toole

When Cameron Diaz slicked semen "hair gel" into her blonde bob in 1998's There's Something About Mary, she elicited a perceptible shift in the annals of modern comedy for women. She hasn't especially revisited the territory since, but does so in her new film, Bad Teacher. The title subject is the entirely unsatisfactory middle-school teacher Elizabeth Halsey, who sleeps at her desk, uses films as lessons, resolutely gold-digs (she has her sights on a rich nerdy substitute teacher, played by her real-life ex-boyfriend Justin Timberlake) and smokes pot (in front of the children).

Diaz is 38 now, still very much on top of her Hollywood game and famously, the very antithesis of a diva. Elizabeth Halsey's appalling antics did not initially impress her. "Thirty pages in, I didn't want to know her and then by 10 pages more I was starting to get her. By the end I loved her. My rule is I have to read it all the way through. It's kind of like the dress that looks terrible on a hanger, and then you try it on and it's the only dress that looks good – after you've said, 'That will never look good'. It was a struggle at first, then I realised how brilliant she was. She wasn't asking for anybody's forgiveness, just being authentic to herself."

Though Diaz hasn't especially revisited outré Mary territory as part of the singularly unique CV she has carved since, Bad Teacher's director, Jake Kasdan, notes that there was no one else in Hollywood who could make this work. "It's impossible to imagine anyone else either in this role or who could have pulled it off." It's even possible to imagine Diaz's contemporaries diving for cover to avoid this unabashedly "out-there" mainstream Hollywood film.

It is not only Diaz's career which is in top gear (she'll shortly return to the London set of the Coen brothers' Gambit opposite Colin Firth and Stanley Tucci). The game of life so important to her is apparently ticking along nicely, too (mostly with American baseball legend Alex Rodriguez in tow). It is perhaps not a coincidence that all her jewellery today, from delicate necklace to chunky gold earrings and bracelet, is golden.

Though Diaz has won acclaim for dramatic roles (particularly in 1999's Being John Malkovich and 2002's Gangs of New York), her personal outlook is lighter and informs most of her career choices. "I think that we all have a little Elizabeth in us. She walks on the dark side a little bit and I think that in life, being human is all about light and dark. They sit very close to each other. You are either walking on one side of the line or the other. If you're lucky you can kind of straddle it and I think I kind of tend to stay more on the light side. I'm a cheerleader for life. I love it, I appreciate it."

She stretches out her impossibly long lean legs on the chair between us. Today they are sporting J Brand cropped jeans and cream, white and tan Chanel stilettos. "Do you mind if I stretch my legs out?" she asks, having already done so and making herself laugh – a familiar, unbridled guffaw. Then she adds quickly, a bit sheepishly, "Oh sorry, you can stretch yours out too."

None of us gets the opportunity to be quite as uncensored as Elizabeth Halsey in the real world. Was that a fulfilling release?

"Of course. It was an opportunity to play a character that I didn't have to be morally responsible for. We all have to follow certain societal standards, and I was able to let that go for a couple of months, leave behind all my moral qualms, then step back out of that at the end of the day and go back to being part of society. I had to play her, I just knew I had to. There was no choice for me."

She also saw it as the perfect role for Timberlake who, in turn, made her job easier during a sexless sex scene, literally and metaphorically, in which the pair bump and grind, shamelessly, gratuitously, while keeping their clothes on. Though Diaz is the actress known to be up for anything on film, even she expresses relief that the scene was with someone she knows well. "If it was anyone else it would have been awkward. It would have been a different scene. But instead we laughed a lot. And because it was him, it just wasn't uncomfortable. We were able to do things I may not have been comfortable doing with somebody else. There is so much trust there, there was no issue. Justin and I know who we are and we know where we stand in our relationship. So there's a comfort in that and a safety."

Were anyone else in the lead role, British actress Lucy Punch, as Halsey's "good teacher" nemesis, Amy Squirrel, might have stolen the film. The unusually generous Diaz (Punch says she was "so supportive and encouraging" the day they auditioned together) insists she actually does. "I really think Lucy does steal the film. Making movies is a collaboration, and it's such an important thing to create an environment where everybody can thrive, everybody can shine. I want everybody to be great and feel confident that they are doing the best work that they can do. I think camaraderie and teamwork is the only way to make movies, or anything in life."

Diaz's own school memories, from Long Beach, just south of Los Angeles, are not particularly happy. "I was really pleased to get out of school; I wasn't really impressed by it. I couldn't sit in the classroom and try to pay attention to what was going on. They call it ADD now, but I don't believe that it's a disorder, just a different way of absorbing information. And I never could really absorb information, just sitting and staring at a person talking to me. I had to be active in doing other things. When I was 16 and working as a model, I lived in Japan for three months on my own and that was a formidable time that taught me that there is more to life than high school and that little microcosm. I discovered the world was bigger than that little world."

She didn't discover acting at school – "I did a term of drama, but it wasn't really anything" – and made her first film, The Mask, opposite Jim Carrey, aged 21. I met her then for the first time, at about 2am on the film's set after its unit publicist begged me to talk to this "sweet model from Long Beach making her first film". "I remember that," Diaz pipes up. "We did the interview on a bench, right?" We did. Does she remember everything since as well? "Oh gosh, some stuff better than others. I've had such an awesome time. I'm so blessed.

"I think that you get back what you put out, and I want to have a good time. I want to bring good things into my life and I think that when you put that out there, hopefully you get it back in some form. But the most important thing is that I'm living. And I think that's just sort of my programme: work really hard and then play really hard."

One part of her programme is a lifestyle so peripatetic that she says she gets itchy feet if she stays in one place too long. "I'm expecting to pack my bags at the end of two weeks because I'm supposed to be some place else. There's a timer in me." Another is therapy. "I've been doing it for 15 years and I thank God. My life would not be half as fulfilled as it is without therapy."

And then there is that body, seen to extraordinary effect in a stunning set of recent photos for Elle and, necessarily, in a predictably soapy car-wash scene in Bad Teacher. Diaz has very obviously ramped up her fitness regimen of late. "I take care of myself, I enjoy taking care of myself. For me, the aesthetic is not the by-product of it. Physically I just am very in touch with my body. I really love feeling strong, feeling capable. I love that feeling in my body. And so when I'm not in that place, I don't feel good. It also helps keep my head clear, and it helps me stay focused on my work and my life and so I just use it as a way of maintaining a certain standard of life.

"I'm sure there are people who look at my body and go, 'Ugh'. But I do it for myself."

'Bad Teacher' is released on 17 June