"A lot of my costars have been sexy guys my age, and so, why not? I'm not going to pretend it's not fun," she explains. "Justin was great -- he had come from doing Friends With Benefits, where he basically had sex every day at work -- and so it was easy for both of us. We just kind of got it on, and then were like, 'That was good!'"
But according to Seyfried, Timberlake, 30, paled in comparison to Megan Fox, her costar in the 2009 horror film Jennifer's Body.
"Jennifer's Body is my favorite movie that I've done. It was overlooked in theaters, but it does have a DVD following," Seyfried tells W. "I can't believe nobody wanted to see Megan Fox and me hitting it."
"They had an extreme close-up of our tongues, and I'm telling you, the thing about the scene is that it's actually really sexy. For a young girl to say that about her own sex scene -- it must be because it's special."
"I think Megan and I kissed really well together," Seyfried adds of her 25-year-old costar. "We have similar kissing styles and it worked. We got it done for the masses, and the masses still didn't show up."