The two stars play a struggling couple in the upcoming drama and shot a scene during which G.I. Joe star Tatum was required to walk around a bedroom naked.
And he was so determined to get a realistic reaction from his onscreen wife, he colluded with the crew to create giant genitalia he could pass off as his own.
McAdams tells Stella magazine, "Channing got the props department to make a prosthetic, um, member. 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!"