Fernando Meirelles, director of Blindness have this habit of making test screenings before going with post-production. While this is not abnormal, it's fairly uncommon.Why does Meirelles do this? Well, he has two reasons. First, he will be able to know what appeals to the audience. Secondly, he will have an insight to cultural differences when it comes to watching movies.
Oh yeah, why don't many film directors do this all the time?
News: Why the director of Blindness likes test screenings