"The whole movie is lovemaking between the characters," Diaz tells PEOPLE. "The crew was awesome and so respectful. It's not a vanity thing. It's about telling a story."
In the film, out July 25, Diaz and costar Jason Segel play a married couple who film themselves trying out every position in The Joy of Sex, only to have the tape go missing.
Which took some, ahem, selective wardrobe decisions.
"We call them modesty garments," Diaz says. "My dresser and I got really intimate. Double-stick tape can be the difference between the whole world seeing your goods!"
And the subject matter certainly provided for a lot of laughs.
Says Diaz: "Sometimes I would stop in the middle of a scene and go, 'Everybody hold on! Do you realize we're getting paid to do this?' We had such a fun time laughing our asses off, having such gratitude that this is our job. We get to be idiots all day long and tell a really sweet story at the same time."