Right before my Birthday, I randomly draw my favorite horror movie to talk about. Session 9 is about a team of asbestos removal workers who are trying to get a rush job done on an abandoned mental institution (the movie was filmed in Waverly Sanitarium, which was an actual empty mental hospital outside of Boston). One guy is a screw up, one becomes obsessed with audio tapes of a certain patient, one has domestic issues and one is a drug user with anger issues. When they begin working in the hospital, all of their worst traits come out.
This is often compared to a low budget Shining but I think the character work is much more subtle. All the various life pressures these guys are under inform all their actions. There is an argument to be made, and I have made it, that nothing supernatural is going on at all. This is just a story of madness and how it infects a normal person's soul.
Some of the line readings are over the top and at least one moment doesn't make any sense without the sub plot about homeless people that was cut out of the theatrical edition. For all its flaws, I think this is pretty much a perfect horror movie in regards to writing, acting and directing. There is only one I rank higher on occasion, but we'll get there. Check out Session 9 if you never have.