Unlike discussion issues that would come after the film, the pre-screening topics should simply introduce the most general themes of that particular movie. This should be easy to do, even if there are no pretenses about the film exploring profound or troubling questions. For example, My Cousin Vinny is a comedy that deals with the travails of two college students from New York who are wrongfully accused of murder while driving through Alabama. Before showing it, a general topic of conversation could be whether anyone in the class has been falsely accused of any crimes, or if they know such a person. Depending on how much interest that generates, you can discuss the criminal justice system in the United States as compared to that of your students' own countries.
Any topic can be useful, so long as it serves as a natural lead-in to the movie.
After an introductory discussion, you may want to quiz your students on the more important colloquialisms listed in the synopsis. Depending on how seriously they want to internalize new words and expressions, and depending on how much time you have, you could in fact use the entire glossary as a springboard for discussing whatever relevant vocabulary is triggered in your head. You might also suggest that students write down some immediate personal reactions after the movie ends, such as whether they were surprised, pleased, or upset. If nothing else, they could note whether or not they liked it, and for what reasons. Another possibility would be to review some of the more difficult vocabulary that popped up in the movie, giving the students yet one more opportunity to internalize the synopses.