Natalie Portman, why must you grow up? You were just so perfectly pixie, skating around and doing the "doo, da-doo, da-doo" from that Lou Reed tune in "Beautiful Girls," or getting teary-eyed at a funeral for a hamster in "Garden State."

Now you're playing a gangsta rapper on "Saturday Night Live" and blowing up British Parliament in "V for Vendetta."

And, Natalie, can we talk about the hair? Does a nice girl go out and get her head shaved?

Meet the radical new Natalie, 24 years old and graduated from Harvard. Out of "Star Wars." And out to change the world, one subversive psychodrama at a time.

In the futuristic "V for Vendetta," which opens this weekend, Portman plays Evey, the politically awakened accomplice to V, a masked and mysterious anti-hero who's either a terrorist or a freedom fighter -- depending upon which side of the dynamite-blast radius you're on.

"I do believe in nonviolence," said Portman, talking to The Post in a Midtown Manhattan hotel room with a sweeping view of the Hudson River. Her hair's half-grown back and gelled straight up in punky spikes.

"But I also am pragmatic. Sometimes not doing something is a violence because you're complicit in whatever injustices your government is engaged in. By being passive, you're being violent."

An adaptation of a graphic novel written in the 1980s, the cartoonish, Nazi-like dictatorship that rules England in the year 2020 thematically clears "V for Vendetta" from real-life terrorism in the post 9/11 world.

(The film's target release date of last November, however, was pushed back after the London subway bombings last summer. Producers insist the delay was required for postproduction special effects and editing.)

But the movie's seditious, anti-President Bush subtext is obvious: The regime that V and Evey seek to undermine controls the masses through wiretapping, television spin and misinformation, using religious piousness for political gain and positioning itself as the protector of a fearful public.

In the "V" world, frightened citizens timidly live life under a "yellow-coded curfew."

"The idea that a big studio action movie could have some substance and could be subversive, that was exciting," said Portman. "But I don't ever desire to make propaganda. The greatest art allows audiences to bring their subjective context to the situation.

"But there are certain specific things in this movie -- for example, the oppression of homosexuality -- I do feel very strongly about. Also the tendency to label in the press. The situation is too complicated to put simplified words on either side. Be wary of oversimplifications."

The same can be said of Portman. Be wary of oversimplifications.

An Israeli-born only child of wealthy New Yorkers, she attended upscale theater camps like Stagedoor Manor and Usden Camp in the Catskills. Yet after early fame in movies like "Heat" and "Mars Attacks!" she attended a public high school in Syosset, Long Island, and remains friends with many classmates.

A millionaire many times over from her three turns as Queen Amidala in the "Star Wars" prequels, she is known to ride public buses around Manhattan.

Before graduating from Harvard with a psychology degree in June 2003, Portman was credited -- under her given name, Natalie Hershlag -- as a research assistant to Alan Dershowitz's "Case for Israel" and had a study on memory called "Frontal Lobe Activation During Object Permanence" published in a scientific journal.

Now she puts that psych degree to work in exploring the inner conflicts of her characters.

"I like the complicated way of looking at issues like violence, and what it takes for someone to become violent," she said.

In other words, Portman is too bright for "hypersexualized, hypersilly girl parts" and too deductive for the lazy Hollywood template of good vs. evil.

At last year's Oscars, she was a supporting actress nominee for "Closer," an examination of the ruins of marital infidelity. Now comes "V for Vendetta," a film that walks the uncomfortable space where terrorism meets heroism.

"Obviously it's hard to make excuses for evil when you're on the receiving end of it," Portman said empathetically. "But when you see the disparities in economics, you can understand the disparities of hope. Some people, they just don't have hope of a better life."

To that end, Portman strives toward a proactive brand of activism in the Third World. As the "ambassador of hope" for the micro-financing organization FINCA, she lobbies governments to provide loans to women for small, start-up businesses.

In the past two years, she's traveled to Uganda, Guatemala and Ecuador, and seen the fruit stands and tiny hotels she's helped enable.

"I barely do anything," she said humbly. "When I travel with FINCA, I'm there to learn. Then I can come back and try to get Washington and London to help out by giving more money. It's a way to help the social and political imbalance by mending the economic imbalance."

This is Portman at 24 -- globally conscious, politically awakened and artistically radical.

But, hold up. It can't be as serious as all that.

"I definitely am politically aware, but I'm also an optimistic person, and I like to go back to personal joys, too," she says.

That's when all the reminders emerge -- those little Natalie Portman things, like tucking her feet up on the couch and rolling up her pant leg as she talks.

Or that mischievous but well-meaning way she messes with your head.

"So, what else should we talk about -- my favorite New York City makeout spots?"

Or the way she stifles a giggle when you nervously kick the table in response.

Or when she talks about celeb gossip in tabloids like, oh, the Post, for example.

"I don't usually read the Post," she says, "But, oh, I was taking care of a friend's puppy the other day and we were using it to make sure she didn't go on the carpet."

Then comes that happy, self-entertained Portman laugh.

Ha, ha. Very funny. The puppy peed on the Post.

Grow up already, Natalie.