I often lead film discussions, but there's enormous value in organising leading them yourself rather than depending on some 'expert' from outside (though sometimes that can be a good thing - I'm still open to invitations!). Here are some brief guidelines. I'll add to this and expand on some of the points at a later date. There's a list of some good films to discuss here.
There are a few criteria to bear in mind when choosing a film to discuss:
Does the film raise good issues which are worth talking about? Are they the right issues for your group?
Is the film suitable for your group in terms of language, violence, sexual content, etc.? There are films in this list which I would use with some small groups in a home, some which would be fine with students, and some which I would never use in church! Don’t just look at the film certificate, look at what the BBFC (or MPAA in the USA) give as reasons for the certificate.
Is your group going to prefer a mainstream film which they know about, or have already seen? Or would it be better to use something unfamiliar, maybe something arty?
A film which runs for 90 minutes or so is great - you can have time for a good discussion and the evening doesn’t feel too long. I think two hours is the top limit in most circumstances. One that runs for 150 minutes may be OK for your group, but many people will be too tired to discuss it much afterwards. Check the running time on DVD, not what it was in cinemas. Cinema projection is at 24 frames per second, but films are transferred onto DVD at 25 fps, which means it will be about 4% shorter. In general, the running times included on my list of suggestions are all DVD running times, but if not, the actual running time will be 4% less.
Keep the discussion focused, starting with some general reflections on how people found the book or film or whatever you are discussing, moving on to explore the ideas within it in some detail, and not forgetting to reflect on how it relate to a Christian worldview. You will find a list of twenty questions to ask about a film here (download the pdf) and a longer list (taken from my book, Focus: The Art and Soul of Cinema) here (pdf file).