Pretty. Gritty.

HEATH LEDGER is driving me home. Movie stars don’t usually deign to drive journalists anywhere, especially long distances that require time, effort and knowledge of the arcane, helter-skelter rules of the New York streets. But this Brooklyn transplant of just five months powers his car -- a hulking blue BMW -- with the leisurely assurance of a cowboy on the range.

He’s slung back in his seat, his long legs stretched out in front of him in ratty jeans, a beat-up black sweatshirt, hood pulled over his cropped, dusty brown hair, obscuring his features. In movies, the high cheekbones slash across the screen. In person, he merely looks indistinct, pleasantly healthy, pleasantly good-looking, curiously unassuming.

“This is my life when I’m in New York. I drive Michelle everywhere,” he says, referring to his partner and “Brokeback Mountain” costar Michelle Williams. “I even got the whole driver thing when she has to run in somewhere. I’ll put on the blinkers. I’ll stand out in front waiting for her. I’ll open the door for her.”

Ledger is wry about his transformation from movie star to chauffeur -- but he’s also not kidding. On the day we meet, Williams is about to give birth to their first child, a girl. They’re also moving to a new house in the Boerum Hill district of Brooklyn. As he cruises up the West Side Highway, the Australia native chats about the rapid life changes of the last couple of months. He sold his bachelor pad in Los Feliz. He moved east. He’s starting a family -- and here the conversation drifts back, as if by some gravitational pull, to the couple’s plans for natural childbirth, the doula who’s tutored them, their foray into a hypno-birthing class, which didn’t work for them. (“I don’t think we’ll be throwing Michelle into a trance.”) They keep the sonograms on the refrigerator.

“The very early ones -- it’s just this part,” he says, gesturing to the bottom half of his face. “It’s just two little nostril holes and then the shape of the lips -- it’s Michelle’s mouth. It’s so bizarre. These little porcelain lips that are exactly the same shape as Michelle’s. It’s just adorable.”

Ledger is in fact so organized he’s written himself a little script for things to remember to say.

“I just need to meet her. I need to hold her,” he says of his daughter (who did, in fact, arrive three weeks after this interview).

Ledger could be just another involved hipster dad-to-be. In a way, he’s shyer about his much more public transformation -- the one that everyone in America is about to see. This fall shows him in a pair of contrasting roles. “Casanova,” in which he plays the raffish title character, showcases his able comedic chops. It’s hardly the story of the real 18th century Casanova -- just an amusing riff on a mythical Casanova, lover nonpareil of women, who falls for the one freethinking woman in all of Venice who doesn’t want him. With an uncredited rewrite by Tom Stoppard, it’s reminiscent of “Shakespeare in Love.”

Ledger’s other film, “Brokeback Mountain,” is the one that represents his full-tilt break from the past -- his evolution from dude to one of the most wrenching and poignant actors of his generation.

A slow route to success

MOST big stars show glimmers in their earliest performances of the talent to emerge. Remember Tom Cruise as the psychopathic cadet in “Taps”? Mel Gibson in “The Road Warrior,” or the hilariously priggish Daniel Day-Lewis in “A Room With a View”? Twenty-six-year-old Heath Ledger is not one of those stars.

Save for several potent minutes in the indie “Monster’s Ball,” there’s little in his resume -- from his American debut in the teen comedy “10 Things I Hate About You” to the Arthurian romp “A Knight’s Tale” and the action-adventure “The Patriot” -- that prepares the audience for the depth and ache of Ledger’s Ennis Del Mar, the cowboy at the center of “Brokeback Mountain.”

Based on an E. Annie Proulx short story, the $13-million film, directed by Ang Lee and shot in Calgary, Canada, is the tale of two dirt-poor cowboys (Ledger and Jake Gyllenhaal) and their love for each other, carefully suppressed and hidden from the world around them. Ever since it won the top prize at the Venice Film Festival this year, the film has been colloquially known in Hollywood as “the gay cowboy movie.” Yet the film’s heroes are a far cry from what’s now become an acceptable stereotype in American pop culture: the liberated, confident and often campy gay man, which populates such mainstream hits as “Will & Grace” and “Queer Eye for the Straight Guy.”

As Del Mar, Ledger emits the kind of loneliness that seeps into your bones like the dampness of a bad winter cold. He’s unvarnished, understated and stoic, fiercely determined to keep his longing and fury and grief pent up for the rest of his life. Catharsis isn’t permitted in his unforgiving cowboy world. As Proulx writes, “Ennis stood as if heart-shot, face grey and deep-lined, grimacing, eyes screwed shut, fists clenched, legs caving, hit the ground on his knees.”

Indeed, so close is Ledger to Proulx’s Del Mar, it’s as if her literary creation got up and walked off the page.

“When I met him, the moment I saw him, that was it. He nailed it,” Lee says. “He’s the person that’s the best to carry that western brooding mood -- elegiac and fearful and violent, all the complexities, all the poetic qualities.”

Today, in his normal life, only traces are visible: a kind of athletic grace, a courtliness under the grunge exterior, a seeming lack of narcissism that one doesn’t usually associate with celebrities or Hollywood.

He doesn’t offer a profound answer about what happened -- how such a performance suddenly popped out of someone whose name used to be eternally prefaced by the word “heartthrob.”

“I just kept growing,” says the actor, who started acting professionally as a teenager in Australia. “I made all my mistakes on film. I never had a black room and a black pair of pajamas to make my mistakes in. I didn’t have acting school or an acting coach. It’s been a long, slow process of making mistakes and changing it.

“I think your performance should mature as you mature as a person. That’s the way it works for me. The greater knowledge you have in life, the greater knowledge you have in how to portray life. That’s just really it.”

Back door to acting

A little earlier, Ledger hunches over a bar table in an old-fashioned pub in Park Slope, not far from his new home. He’s explaining the tattoos on his forearms -- both homages to the women in his life. Across one wrist is the word “Kaos” -- which stands for Kate, Ashleigh, Olivia and Sally, his mom and three sisters. (“They’re so not chaotic!”) On the other arm is the phrase “Old Man River.” Williams wrote it there, and he’s had her handwriting tattooed.

“There’s something eternal about the phrase. Now I’m Old Man River. Life is about to speed up from this point on. It has nothing to do with show tunes.”

“It’s a little personal,” he says shyly, and then laughs.

Ledger grew up in western Australia, in Perth -- the “most isolated city in the world.” His father designed racing engines for cars, but his parents split up when he was 10, and he and his sisters went back and forth between the two homes, moving every two weeks. His ease on horseback -- and directors’ penchant for casting him as someone who rides -- might owe something to the fact that his stepfather owned a farm. “I used to chop wood. That was my chore. I grew up around horses. Funny enough, I didn’t start to ride horses until I had to. Then I couldn’t get off horseback.”

He says he essentially fell into acting at the instigation of his sister’s agent, who got him cast in an Australian TV show. Other TV series followed, on which he played a gay cyclist (“Sweat”) and a medieval prince (“Roar”). Ledger recalls that after one TV performance, he even asked his mother for reassurance. “It was terrible. I was terrible in it. Back then, I couldn’t act to save myself. I even remember asking my mom, ‘I’m not very good in it. I’m really bad.’ I was waiting for her to say, ‘No, hon, you’re not, you’re just fine.’ ”

But no.

“She was like, ‘Well, ... you know ... it doesn’t matter.’ ”

There was an upside to his realization, he recalls. He at least knew that he was bad. “That’s a step forward. I can understand what’s good and bad. I can begin to study what I’m doing wrong. I’m blinking too much. I’m not really listening to them. I’m just saying the lines. I started to change it myself.”

The parts followed -- and like many in Hollywood, he alternated popcorn with more arty fare. Unfortunately, the more challenging films -- pricey epics like “The Four Feathers” and “The Brothers Grimm” -- wound up more ambitious than successful, either artistically or commercially.

“I’ve known Heath for a long time,” says James Schamus, Lee’s producing partner and now co-president of Focus Features, which is releasing “Brokeback Mountain” on Dec. 9. “I always thought he was underserved in a lot of the roles he did. We give all the credit to ‘Monster’s Ball.’ There are two minutes in that film that show he’s among the best actors of his generation. All we had to do was extend that two minutes into two hours. Just letting the camera roll.”

Ledger wasn’t even supposed to be in that film, but when his pal Wes Bentley dropped out at the last minute, he asked Ledger to cover for him so as not to derail the production. Ledger’s harrowing performance, as a son withered by his father’s contempt, was the first indication that the Aussie might be more than simply a handsome screen presence.

Ledger knew from the moment the Larry McMurtry-Diana Ossana script for “Brokeback Mountain” dropped in his lap that he wanted to play Ennis. He even told the filmmakers he’d fly to China just to meet Lee.

“I enjoyed the stillness, and I enjoyed the lack of words on the page. There was so much information about him in the short story, I knew how to play those silent moments.”

Lee is famous for working closely with his actors in preproduction and rehearsal, then barely speaking to them once filming has commenced. Not that this method was a hindrance for Ledger.

“He’s very meticulous,” Lee says. “After we work on the scenes, in terms of rehearsal, he will have a very solid idea of what he wants to do. He’ll turn himself into the zone, the zone I helped create, that movie world. Somehow he’ll get in there and live there.”

In playing Ennis, Ledger absorbed his utter desolation. “The whole shooting experience for me was incredibly lonely,” he says. “Whether or not Ang created that environment for me to work and live in, or I created it for myself -- it’s a lonely story, so it’s hard not to take it home with you and feel lonely.”

He admits that both he and Gyllenhaal were “very, very nervous” about the gay love scenes -- which are straightforward and unusually frank. (Indeed, there were those in Hollywood who even in this day and age thought that the two young heartthrobs could potentially alienate their teenage fans by playing gay.)

“It’s not something that we’d searched for in everyday life. But we’re pretty sensible people, Jake and I. We realized we’re just two people, “ says Ledger. “We realized it’s necessary for the story -- the level of intimacy had to be portrayed to increase the level of heartache for the story. It’s easy to say it was difficult and hard, but it’s really awkward having to do a love scene with anyone -- whether it’s a guy or a girl. There’s a guy with the boom standing over you. It’s always awkward.”

‘Brokeback’ recovery

HE recovered by flying to Venice for six months to play the title role in “Casanova” -- as breezy and romantic as “Brokeback” is spare. It was a seat-of-the-pants production, with a totally new script delivered just two weeks before the start of shooting.

For director Lasse Hallstrom, a different kind of Ledger showed up. “He wants a loose atmosphere. He wants to be spontaneous and improvisational. Everything is geared toward that,” Hallstrom says.

“It was basically a working holiday. An elaborate extended period of unwinding, so I could unravel myself from ‘Brokeback Mountain,’ ” the actor explains. “It was really fun, just working in Venice. The city should only exist in your dreams. Getting picked up every morning in a boat. There are no camera trucks; there are camera boats. There are no trailers to hang out in. There are the streets and chairs. It’s all very communal. All the cast would sit around and play dice games.”

As he nurses his beer, Ledger bounces back into his genial persona, his Australian-tinged voice deep and somehow reassuring. But clearly, that’s not all there is to him.

“I don’t really like to do the same thing twice,” he finally admits. “I like to do something I fear. I like to set up obstacles and defeat them. I like to be afraid of the project. I always am. When I get cast in something, I always believe I shouldn’t have been cast. There’s a huge amount of anxiety that drowns out any excitement I have toward the project. Pretty much any time I’ve signed on to a movie, I’ve tried to get out of it.” He goes so far as to call his agent with his plans to escape. “He knows it’s a routine. I know it’s a routine. It feels like it’s necessary to put myself down. It inspires me to focus more and work harder.”