Elfman is part of the ensemble cast of the new romantic comedy. She filmed it right after birth of her second son and joined the group after everyone else had been there for a month.
"I'm like coming in to an already established group, and I'm supposed to have this lifelong chemistry as Justin Timberlake's sister," Elfman said. "He was so cool, and I was so happy to be out of the house."
While the film brings up the possibility that men and women can be intimate and still be just friends, Elfman says she's not buying it.
"I've never engaged in a friends-with-benefits lifestyle, or attempted to, so I can't speak firsthand on that, but it doesn't seem like a workable lifestyle to me," she said.
Elfman has been with her husband for 20 years.
"It's the friendship that gets you through the hard times. Sex can make you feel better during times of stress, but actually getting you through those times, it's a friendship," Elfman said.
Elfman pointed out that she and her husband may have tough times, but the marriage is still "so awesome.
"You're exchanging these experiences, and you're part of each other's growth, and you're part of each other's gray hair," she said.
Elfman appears with Timberlake and Mila Kunis in the R rated "Friends with Benefits," in theaters this weekend.