The star insists she felt she had no choice but to bare all in the famous sex scene and admits not only was she acting but she was also pretending not to be uncomfortable.
"That's kind of good acting - where you're really in a situation you don't want to be and trying to make it look comfortable and authentic," Sean says in new documentary "Skin: A History of Nudity in the Movies", which debuts as a video on demand on Tuesday, August 18.
She also reveals Costner was even more nervous than she was - and he got to keep his clothes on.
"I remember turning to him and saying, 'You're dressed! What are you talking about? Get it together, would ya?'."
Looking back, Young is convinced the scene would not be shot as it was today, adding, "(Director) Roger Donaldson at the audition when he finally offered me the part, he said, 'Lift up your shirt. Let me see. I want to make sure you don't have any scars or anything on you'. So I was like, 'OK,' and lifted my shirt. It was really ridiculous on a certain level.
"It's not like you could say to the producers, 'I love this part but I don't want to do the nudity'. You had to do the nudity. That's what they were banking on as part of their sexy project. I remember doing the love scene (in the limo) and thinking how retarded it was that I was the only one undressed."