"It was so weird, it didn't even feel like we were doing a Twilight film. I was like, 'Bella! What are you doing? Wow! What is happening here?" she told Glamour magazine.
"It was very surreal. We got rated R. They re-cut it."
Pattinson also dished about the between-the-sheets action in the new flick.
"The sex scenes were difficult," he said "The set was crazy and you have about 20 people in the room and someone saying things like, 'No, Bella would not have made that noise. Can we do it again?'
"It was very charged, but a very tender and sentimental scene. It's not a scene from the book, so everyone was nervous about doing it right. A ton of other people were nervous. And, Kristen and I were both worried about the shape of our bodies would look on camera. We felt vulnerable."
Pattinson also joked that he and Stewart may already be married.
"The wedding scene's funny because we used a real priest," he said during a press conference in Stockholm, Sweden, last month. "So technically we are already married because he did all the things you would do in a normal ceremony. So we don't have a civil union in law, but I guess in the church we are actually married."
Back to the derriere.
"How do you cut the crack out?" asked Ellen.
"They just painted over it," Pattinson said.
Replied Ellen: "It makes you look like you don't have a crack. They cover it so there is no crack on your butt? You just look like one solid cheek?"
"You're allowed to show cheek," Pattinson explained. "You can't show crack."
Pattinson, including Taylor Lautner, Kellan Lutz, Ashley Greene, Nikki Reed, Jackson Rathbone, Peter Facinelli and Elizabeth Reaser also appear on the episode, due to air on Friday in the US.
Preview the episode here.
Meanwhile, Pattinson, Stewart and Lautner hit the cover of Entertainment Weekly and dish on how the movie's PG-13 rating limited the level of passion they could show.
"There's a version where it was really intense," says Pattinson.
Says Stewart: "Cinematically, cut all together, it's awesome. But we didn't really do that scene."
"And when we did, they kept telling us to stop," adds Pattinson.
Check out more from their interview here.
* Have you seen Breaking Dawn - Part 1? What did you think? Spill your thoughts below.