Nymphomaniac, a movie directed by Lars von Trier (he did Melancholia), will use a groundbreaking digital editing technique to show graphic sex scenes on the screen. For the non-explicit portions of the sex scenes, you'll see celebrities and actors you recognize. For the penetrative portion, you'll see the body doubles doing it. The difference is you won't know the difference because the film will digitally stitch the two together.
Producer Louise Vesth said:
“We shot the actors pretending to have sex and then had the body doubles, who really did have sex, and in post we will digital-impose the two. So above the waist it will be the star and the below the waist it will be the doubles.”
It's a lot like how body doubles are used now in movies but presumably more seamless in its execution because of technology. So not unlike the CGI nipples we've seen recently in movies (though I guess a lot more explicit than that). Von Trier, who's done movies like Antichrist and Dogville, will also use "experimental graphical elements in the two Nymphomaniac films, using double exposures and imposing words and symbols over the action as part of his storytelling."