This is a neat reminiscence on working with the late Sidney Lumet. Henriksen (best known as Bishop from Aliens, probably) was an FBI agent in Dog Day Afternoon, a character who appears to be just above extra for most of the film, and turns out to have shattering significance in the last moments. I like the idea of him rehearsing the movie in sequence with all of the cast together. It might be a time-intensive way of making a movie, but it helps explain why DDA has so much cumulative power.
Speaking of cumulative power, this is an earlier exhibit of Lumet's craft, from 1957's 12 Angry Men