I never got the impression that either one was the “main” one. An open file versus a closed file interview have two distinct purposes. In my far from infallible opinion, an open file interview is to get a better understanding of your extracurriculars, volunteering, shadowing, etc or to tie up loose ends, gaps, or red flags from your application. I feel like the closed file interview is more suited to learning more about one’s core character traits and personal disposition. Neither is the “main” interview.