Shared with you
The filmmakers used a body double for the scene of Natalie Portman diving into the lake. The official reasoning was that it was an insurance issue, but Danny McBride heavily implied that it was because Natalie Portman didn't want to bend over in the thong. But Portman said in an interview that she didn't want to dive into the water because it was too cold.
Although the film was written by Ben Best and Danny McBride, the dialogue is heavily improvised. Director David Gordon Green said there was never a script used on-set. Only the plot outline and written notes were used.
The premise for the movie evolved from a frivolous game David Gordon Green and Danny McBride used to play to while away on film sets. Each would take turns in suggesting a title for a film, and the other would try to concoct a viable storyline. When the title "Your Highness" was put forward, the premise was, "It's about a Prince who gets stoned and fights dragons."
During filming, James Franco flew back and forth between New York City and Belfast, because he was attending full-time classes in college.
Natalie Portman admitted she only did this movie because there was some uncertainty if Black Swan (2010) would get green-lit by a major studio. She signed on to this film for a paycheck, and use it to help finance Black Swan herself, should the need have ever risen. However, Black Swan got green-lit, and went underway fairly easily, and Natalie got stuck doing this film having already signed on.