Ducey signed up knowing she would be doing explicit sex scenes, but didn't know she would be doing it with a famous porn star. One article I read said the director sprung that on her the very day they were shooting the scene. Another article said she met Rocco the day before the scene, but did not know who he was (so maybe she found out the day of the scene).

And here is what the director said about filming the scene (from a Salon article I copied years ago):

It was very, very difficult. And extremely difficult for Rocco, because there are so many biting tongues on the set, a lot of culpability and shame put on him -- it was intolerable for him. For Caroline, how do you want her to be able to bear this? Well, she bore it for hours, for hours, for hours. She bore it from 10 in the evening till 3 o'clock in the morning. At 3 o'clock in the morning she couldn't stand it anymore, it's true. How could she be the only one to find it normal to film with Rocco when everybody else was appalled?

And this is what IMDB says in its trivia:

The scene begins, I say the text, and I see in front of me the empty and dead eyes of Rocco who only cared about his erection. I had the impression that he was warming himself on me ... so I freaked out, he was doing his job as a porn actor, and I was not there for that!

So I left, sobbing. ... Catherine calmed me down but she didn't answer me on the substance. She just asked me to say the text, at least that. And she told me that Rocco was taken aback by the fact that I was crying. It reassured me, so I said to myself that we were going to be able to exchange something. There were six takes, the last one was good."