''I'M ONE piece of a puzzle,'' Kirsten Dunst says. She's not being modest or enigmatic - just matter-of-fact about the way filmmaking works. She is speaking from Greece, where she is in the midst of filming The Two Faces of January, an adaptation of a Patricia Highsmith thriller. Searching for a word, she apologises that her ''brain is at half-mast'' but she's quick, thoughtful and relaxed.
Dunst, 30, has made more than 40 movies, moving seamlessly from child performer to teen movie star to romantic-comedy lead to an actor with apparently effortless clarity. Her first movie appearance was an uncredited role, at the age of six, in Woody Allen's segment of New York Stories; she was Tom Hanks' daughter in The Bonfire of the Vanities; and Amy in Gillian Armstrong's Little Women.
In Interview with the Vampire, at 14, she seemed simultaneously youthful and ageless as a spookily precocious member of the undead, opposite Brad Pitt and Tom Cruise. She has made films that have been box-office hits and critical favourites, such as the cheerleader movie Bring It On and the smart and soulful superhero movie Spider-Man (she has called Spider-Man an ''independently minded blockbuster'').
There's a lightness and a transparency but also an earthiness to her performances, whether she's the love interest in Spider-Man or the office assistant in Eternal Sunshine of the Spotless Mind, or the dream girl at the centre of Sofia Coppola's exquisite The Virgin Suicides.
Last year, in Lars von Trier's Melancholia, for which she won best actress at Cannes, she was extraordinary, playing a fragile, volatile bride at a disintegrating wedding that takes place just before the end of the world. She remembers, before reading the script, being excited by the thought of working with von Trier, but wondering if she was going to be ''comfortable with the material'' and finding, to her relief, it was a role she knew she could play.
Her latest movie is about a bridal juggernaut with a very different focus. Bachelorette is an off-kilter, hyperactive comedy about three women and a long, dark night of cocaine and chaos on the eve of another woman's wedding. It sounds like a project constructed to cash in on the success of last year's Bridesmaids, except it precedes that movie by several years: it's based on an off-Broadway play by Leslye Headland, who wrote and directed the adaptation. ''She's a fireball,'' Dunst says of the director, ''energetic and hilarious.''
Dunst's character, Regan, is a crisp, composed young woman who looks as though she has everything but, Dunst says, ''is not getting what she thinks she wants''. Regan is a Princeton graduate, an elegant blonde micromanager with a job that makes her look virtuous and a boyfriend studying pre-med. But when someone she has known for years, the good-natured Becky (Rebel Wilson), asks her to be her matron of honour, Regan is a seething mixture of compliance and why-not-me resentment. There are things to which she assumes she's entitled, and a wedding is one of them.
Regan's friends and fellow bridesmaids are in a different league: Gena (Lizzy Caplan) is partying like it's 1999 and Katie (Isla Fisher) takes ditziness to new heights. Dunst gives Regan a tightly wound intensity, a kind of desperate efficiency: she is monstrous, but Dunst finds a way to expose her wounds, to make her seem vulnerable as well as appalling.
Yet Dunst is certainly not indulgent towards her character when she talks about her. She thinks the women of Bachelorette need to grow up, that they suffer from a serious sense of entitlement. And Regan is the most self-centred of them all. ''I've met type-A girls like this,'' she says. ''She's just go, go, go and me, me, me. She could definitely use some yoga, some meditation, some spirituality in her life.''
What she liked about the project was that it felt different. She read the script and the play script as well (''it was much darker; it was about addicts, basically''). The film version is lighter but there's still hectic, catastrophic behaviour, and testing of taboos and audience tolerance. ''You haven't seen girls behaving like this in a comedy. In a drama, maybe. But this is a new kind of thing, don't you think?''
After Melancholia, she felt she needed a change of pace. Despite the range of roles she's taken, she's wary of being typecast, of being seen in a certain way.
She is also wary of being too specific about the nature of choice. ''It's such an intuitive thing,'' Dunst says.
What guides her decisions about roles is hard to pin down. It's not so much that she is clear about what she wants to do, it's more about ''knowing what I want to explore in myself next''. That doesn't always make the difference. ''Sometimes I read something and it excites me. I might not know what it is - I might not figure it out until after the film.'' The final product, she emphasises, is out of her hands. ''I'm a piece of the puzzle. I'm not the editor, I'm not the director.'' All sorts of things can affect the way a film turns out. ''Music can ruin a movie. Anything can.'' So she has to focus on what takes place on set. In fact, she says, ''the experience of making the movie is my career. If it does well, or it's good, that's icing on the top. Because you never know - you can have the best of everything and the movie can be total crap. Or you can think, 'This movie's going to be terrible', and it's great.''
Down the track, Dunst has plans to take another role in the process. She hopes to work with one of the producers of Bachelorette, who has become a friend. ''I want to produce big, funny movies. Small ones make no money,'' she says, with a raucous laugh. ''And I love comedy. That's what I want to watch on the couch after a long day at work.''
Bachelorette was a low-budget movie, made very quickly and marketed in a particular way. After a positive reception at Sundance, it was released on iTunes in the US before it went into cinemas. It was a runaway hit and recouped a good portion of its $3 million budget this way.
For Dunst, it takes a female writer to create these kinds of roles and films. She mentions Bridesmaids, written by Kristen Wiig and Annie Mumolo. She was a big fan of Charlize Theron's performance in Young Adult - written by Diablo Cody - as a woman who never got over being prom queen. ''I love delusional woman parts, like To Die For [in which Nicole Kidman played a dangerously ambitious would-be celebrity]. I love women who are just living in their own world. Those are so fun to play.'' The important thing, she says, ''is to be truthful in your delusion and not to judge the person you're playing''.
You can play extremes, she says, without living them. She talks about working with Viggo Mortensen, who's also in The Two Faces of January, and notes that being an actor ''is more about accessing your feelings than having to live a tormented life. The best actors I have met have been very kind, down-to-earth people, but they can access things, like a child.''
■Bachelorette opens on November 1.