LOS ANGELES — Brad Pitt is about to crush a dog.

"Hey, that's a living creature," David Fincher calls out to Pitt, who is zipping around the director's cavernous Hollywood office on a Segway, a stand-up, motorized scooter. "Try not to kill the living things in here."

Pitt has peeled into the converted bank building with Lenny, a playful bull terrier that serves as office mascot, chasing the star. Pitt corners Lenny in a dead-end hall. The dog freezes, startled to go from predator to prey.

"I got it, I got it," Pitt says, reversing the scooter a few inches from Lenny's snout and whizzing past Fincher to terrorize human employees. "You worry too much."

Fincher shakes his head. Three movies together, scores of stunts, years of ducking paparazzi, "and this is how he's going to kill himself."

Fincher had better hope not. Since Pitt and the director met 15 years ago, they have become close friends — and crucial components in each other's professional lives.

Fincher provides Pitt gravitas. He may have received an Oscar nomination for his role in Twelve Monkeys, but Pitt is the first to acknowledge that his dramatic chops are more recognized in Fincher's Se7en and Fight Club.

Pitt, in return, bestows Fincher inordinate clout. For a guy who has never made a blockbuster, Fincher is able to make the movies he wants, glum endings and all, thanks in part to his friendship with the most famous actor on the planet.

Soon, they could be introducing a new dynamic to their relationship: Oscar consideration. Their latest venture, The Curious Case of Benjamin Button, which opens Christmas Day, has topped many prognosticators' Academy Awards lists, including for acting and directing.

Not that either walks with Oscar swagger, particularly when you put them in a room together. They may be all business on screen, but talking with them in private is a little like trying to study at a frat party.

Straight answers are out the window. They can be crude and cutting and speak in shorthand that puzzles even colleagues.

"Sometimes you have to interrupt them just to get them to speak English," says Taraji Henson, who co-stars as Pitt's mother in Button. "They're like brothers — opposite in the way they act, but they understand each other the way families do."

If that's the case, Pitt, 44, is the goofball younger sibling, the Costello to Fincher's Abbott.

"Why's he called Lenny?" Fincher, 45, asks Pitt as the dog follows the two men into Fincher's office, where a breakfast of bagels and cream cheese awaits. "What's he named for?"

"He's named because it's easier for dogs if they have names," Pitt says. "And I believe Lenny comes from his owners, who named him."

Fincher drops heavily in an office chair, amused that the cracks will begin this early in the day.

"It's amazing," he says, "that we've ever gotten a movie made."

Not the dashing, slick Pitt

Button almost wasn't made. Based on a 1922 story by F. Scott Fitzgerald, the idea bounced around Hollywood for nearly two decades. The tale of a child born with an old man's body who gets younger as he ages stymied studios uncertain how to flesh out the short story. And creating a decrepit infant who becomes more vibrant over time was a special-effects nightmare.

When screenwriter Eric Roth got hold of the script, it turned into a Forrest Gump-meets-Tim Burton saga, and Fincher was sold. "We had the story," Fincher says. "I figured the special effects would come."

Still, both director and star would have to wade into unfamiliar waters. Pitt would have to get ugly, Fincher happy.

Pitt has played unsavory before, a remorseless serial killer in Kalifornia and a deadly gunslinger in The Assassination of Jesse James by the Coward Robert Ford.

But Pitt's fame has always stemmed from playing versions of himself: dashing, grinning, quick with a quip. There's a reason his biggest films —Mr. & Mrs. Smith, the Oceans franchise — showcase the slick Pitt.

To play Button as a senior citizen, he had to be caked in makeup and wear prosthetic noses and scalps. For much of the movie, he abandons his own body entirely, providing a face and voice that are digitally imposed on other actors including a dwarf, a disabled man and children of all ages.

Fincher had to abandon the brutality that has marked films such as Zodiac and Panic Room.

"I think he had to display the side he's shy about showing," Pitt says. "The side that's a father, that's really very sweet. He doesn't like to let it show, but it's there."

Perhaps, but Fincher also is known for being a tortured filmmaker. "If he doesn't get the shot he wants, he's physically pained," Henson says. "His shoulders hunch. He gets sweaty and flushed. He'll say things like, 'I'd have the perfect shot if that (expletive) extra would stop looking in the camera.' "

About the only thing that relaxes him, she says, is Pitt.

"He's David's muse, partly because he's so easygoing," she says. "He'll crack a joke, or they'll start ripping on each other, and everything is relaxed again."

Still, there are hurdles to filming a Brad Pitt movie. Namely, Brad Pitt.

Up to two dozen photographers shadow him wherever he goes. (For this interview, he arrived on his nondescript Ducati; the helmet allows him anonymity on the street to elude the "razi," as he calls them.)

The set was placed on "Pitt Patrol." There were body doubles and decoy cars. Pitt's driver never crossed an intersection when it turned green. He waited for it to turn yellow to leave law-abiding photographers at the red light, though few obeyed the signal.

The swarm of attention made the decision to film in New Orleans an easy one. Button became the second Hollywood feature film, behind Denzel Washington's Déjà Vu, to film in the city after Hurricane Katrina.

The locale provided Fincher peace to shoot in relative quiet.

"After the storm, it was like someone had taken a squeegee," he says. "I mean, there was no one around, not even birds. But there was still this spirit there that people weren't going to be defeated. I consider this movie a love poem to New Orleans."

Pitt, too, fell in love with the area. He bought a house where he, Angelina Jolie and their six children still escape to "a sense of normalcy, if there's such a thing."

"It's the only place where my family can have a sense of privacy," he says. "People there have other things to worry about than all this silliness."

On the same ending page

Not that Pitt is opposed to a little silliness. After all, he discovered Fincher after a drunken night in Tijuana.

The actor says he felt like seeing a movie "after a night of utter debauchery" in Mexico in 1992. He settled on Alien 3, Fincher's feature-film debut. The movie's malevolent tone and (spoiler alert) stunner conclusion — the young director killed off Sigourney Weaver's character, Ripley — earned the movie abhorrent reviews. It grossed $53 million, the worst in the franchise.

Pitt loved it.

"I remember walking out of the theater thinking, 'OK, that was not what I expected,' " Pitt says. "That wasn't a Hollywood ending. It really stuck with me."

"Maybe it was the hangover," Fincher jokes.

Soon after, Pitt got hold of the screenplay for Se7en, Fincher's cop drama about a serial killer who murders based on the Seven Deadly Sins.

Pitt immediately signed on, with one caveat: The studio could not change the film's final scene (spoiler alert 2): Pitt's character executes the killer.

Pitt and Fincher found they clicked on set, despite stark differences in style.

Fincher loathes being in front of the camera. He rarely grants interviews. He tends to dress like he's settling in for a Sunday afternoon of football. The divorced father of one lives only a few blocks from Pitt in the Hollywood Hills, but Fincher often drives separately when they socialize to avoid shutterbugs.

Pitt, by contrast, seems at ease with any lens. Considered one of the most patient stars with paparazzi, he's perpetually braced for an unexpected photo op. Even when carousing with Lenny outside in 85-degree temperatures, he never removes his red felt fedora.

But for all their differences, they discovered they shared a similar sense of humor and irreverence.

After Se7en wrapped, Fincher wondered what should be done with the bloated mannequin used to re-create the murder of a 400-pound man.

"Let's save it for the party at the premiere," Pitt told him. "And fill it with bean dip." Studio executives nixed the idea.

But when they tried one last time to change the movie's ending, Pitt and Fincher's friendship was permanently bonded.

"They tried all kinds of things to change our minds," Pitt says. "We wouldn't budge. David isn't afraid to use an ending that works, even if it isn't the one you want. That's why I trust him. If he wants me to do a movie, I say yes first, then find out what it is."

Fincher is getting uncomfortable with the public praise. As the morning wraps up, he heads back to his desk. There are still finishing touches left to make on the film.

Pitt is ready to play again. He hops out of the chair and heads for the scooter, looking for Lenny for Round 2.

"I'm trying to get David to be in a gang with me on these things," he says. "He thinks we'd look like a couple (jerks)."

He turns on the scooter. "But I can change his mind."