James Franco says his recent Instagram activity isn't really about showing off his body.  Instead, he explained on Thursday's "Late Show with David Letterman" that it's about understanding what will get attention on social media. 

Franco said he likes Instagram, and things like it, because it gets people to notice the things he's doing.  But he's learned that he'll get a lot more notice if he posts a picture of himself in his underwear, than he would if he shares a picture of his new book or just his face.  

But Franco has been doing more than just posting images to Instagram.  His new movie, Palo Alto, opens in select theaters this week.  It's based on a collection of short stories he wrote, but he says he didn't originally plan to be in the film.  The director, however, asked him to play the part of a teacher who has an inappropriate relationship with a student.  And while Franco says it wasn't a role he was dying to play, he agreed to do it to help support the movie.

Check him out on "The Late Show" below!