The Mamma Mia! star had to pucker up during auditions for the two young male leads, which eventually went to Shiloh Fernandez and Max Irons - but Seyfried didn't enjoy the experience.
She tells BBC's Newsbeat, "It's gross actually, I felt so dirty afterwards. I was required to make out with each of them - and you don't know what they're going to do. It was bizarre.
"I wanted to give each guy the best. I didn't want to give Shiloh more than I gave Max or any of the other guys. I was trying to be so good. I was just exhausted - so was my mouth."