"He's showing his penis," the 42-year-old actress says bluntly while discussing the reason behind the book-turned-movie's box office success.
Garner's response to her husband baring it all, "'You're welcome.'"
She added jokingly, "I just try to consider myself a charitable person and I wanted to give back. You give me so much, I wanted to give something back to all of you."
As for how Affleck, 42, brought up the scene to his wife, Garner admits that the conversation was casual. "It wasn't a discussion, it was like, 'Hey, at work today [director David] Fincher talked me into coming out of the shower,'" she recalls. "And I was like, 'Oh, cool, well, I hope he had on a wide lens.'"
Affleck has also cracked some jokes about his Gone Girl nude scene, telling MTV News, "The penis is in there! It’s IMAX penis! You’ve gotta pay 15 bucks to see it in 3D… it’s better in 3D."
Can't get enough of Jen and Ben? Check out ET's very personal chat with Affleck: