"Channing got the props department to make a prosthetic, um, member," the 33-year-old actress told U.K. magazine Stella about Tatum's mischievous plan for their nude scene.
"Literally a fake penis. I didn't know what to do -- I thought, 'Is that the real deal?' It was very realistic. It was ridiculous, the scale of it!" she said.
"I really didn't have faith in her acting, to be totally honest. I wanted to get a real reaction from her, because it needed to be good," Tatum joked to Access Hollywood. "I made sure that I had [a prosthetic] that was down to my shin and that take is in the movie."
"The funny thing was she looked back [at it.]"
Luckily for McAdams' current boyfriend Michael Sheen, it was not the real deal. She told the mag that regardless the size of his junk, she and Sheen do whatever it takes to keep their relationship intact.
"Michael and I never spend more than three weeks apart -- we rack up a lot of air miles -- but you have to be quite adaptable in this business whether you are in a relationship or not," McAdams said. "Trying to establish roots somewhere is a bit of a joke."
British-born Sheen lives in Los Angeles while McAdams maintains her residence in Toronto, Canada, where she shares a home with her brother.
"I feel very much at home [in Toronto]," she explained. "I ride my bike everywhere. I have my favorite restaurants."
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