I came across a movie about 10 years ago that was mainly two people sitting at a table and discussing suicide (among other topics). Most of the suicide parts of the conversation consisted one one person trying to convince his friend that suicide is never the answer. What this person didn’t know was that his friend had already poisoned himself and was sitting there dying slowly as the conversation continued.
What impressed me most about this movie was that it was done entirely in one take. No cut scenes, no fadeouts, nothing.