I was just thinking about this topic the other day as I watched "Three Women" by Robert Altman. I loved the ending of that movie, which relied on an unpredictable and totally unrealistic and inexplicable switch, but I thought how hard that would be to write effectively. That movie depended so much on the emotions created by visual imagery -- and while you can "write" visual imagery, it's not just quite the same as seeing it.
As Renee notes, the characters and plot of a movie are only half realized by script and then the actors, music, cameras shots etc. take up the slack. I have never tried using the plot and characters of a movie to write a story, but I have been inspired by specific movie images in my writing, and I sometimes adapt image-driven folktales which also requires one to flesh out plot and character. I'd like to experiment with movies, specifically switching plot and characters relationships at the ending as Altman did in "Three Women."