From WENN.com

The Oscar-nominated Milk director will appear alongside real-life X-rated actor James Deen in a scene in the film, according to screenwriter Bret Easton Ellis, who has tweeted a shot of Van Sant on set.

The filmmaker will portray a character called Dr. Campbell in the movie.

It's not Van Sant's first acting role - he also had uncredited cameos in his films Finding Forrester, Psycho and My Own Private Idaho and appeared in TV series Entourage in 2008.