Sienna & Hayden's Sex Scene in Factory Girl is "The Real Thing"

Sienna Miller and Hayden Christensen treat us to some utterly convincing lovemaking in their new movie, "Factory Girl." And it's no wonder: We hear the costars actually coupled on camera.

"It's not simulated," an insider tells us. "They're really doing it."

In the movie, Miller plays doomed Andy Warhol protégé Edie Sedgwick. Christensen plays a folk rocker modeled after Bob Dylan.

It was during the film's Louisiana shoot that Miller ran into another squall in her stormy romance with Jude Law - and turned for comfort to Christensen, the "Star Wars" heartthrob.

"They spent about a month hanging out," says one pal of Christensen. "But then she decided she didn't want a relationship. Hayden was devastated. He really fell for her."

Added a friend of Miller, "Sienna wanted to try to make another go of it with Jude. But, again, it didn't work out. At the end of last summer, she and Hayden ended up in Toronto for more shooting. They hadn't talked in six months. But it turned out to be a great reunion."

Apparently. When it came time to shoot the love scene, word is the former lovers dispensed with the flesh-colored socks and pads favored by actors on less familiar terms. And, once the cameras were rolling, the two 25-year-olds fell into old habits.

At the movie's premiere on Monday, director George Hickenlooper would tell us only: "Sienna and Hayden grew close during the filming. It was an emotional experience for all of us." As for the sex, he said, "We tried to portray it tastefully."

And was congress actually in session during the shooting?

"I can't comment," Hickenlooper answered. "You'll have to ask Sienna about it."

Miller left the Chelsea Hotel after-party before we could ask about the love scene - and where she now stands with Christensen, who wasn't at the premiere. Yesterday, Miller's publicist said the sex wasn't bona fide: "She's just a really good actress."

But now we're wondering if that "really good actress" forgot a little something Monday night. She'd changed out of the divine white party dress she wore on the red carpet at the premiere into what looked like support hose for the after-party. Even Sid Vicious, who supposedly haunts the Chelsea Hotel's elevator, would've been spooked.