"It's not simulated," the insider said. "They're really doing it"
Miller's publicist said the lovemaking was merely good acting.
"When you do a love scene there is a minimum of five people in the room. There's no real sex and the scene proves she knows how to give a good performance," her rep told TMZ.com.
In the movie, which opens this weekend, Miller plays the beautiful, rich and ultimately doomed protégé of Andy Warhol, Edie Sedgwick. Sedgwick died of a barbiturate overdose in 1971 at age 28.
Christensen plays a folk rocker modeled after Bob Dylan, with whom Sedgwick is rumored to have had an affair.
The News reports that Miller had a romance with "Star Wars" actor Christensen while on a break from her ex-boyfriend Jude Law — but then jilted him.
"They spent about a month hanging out," a Christensen pal told the News. "But then she decided that 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."
It was during this "reunion" that the two former lovers, both 25, reportedly did the deed for the world to see.
At the movie's premiere on Monday, director George Hickenlooper told the News: "Sienna and Hayden grew close during the filming. It was an emotional experience for all of us."
As for whether the sex was for real, he said:
"I can't comment. You'll have to ask Sienna about it."
At the premiere's after party, Miller changed into a strange black outfit that looked like granny panties worn over black tights.