Been reading here and there. Thought I'd better jot this down while it's in my head.
Set Piece in film are the thing everyone talks about after they've seen the film. Hitchcock called them crescendo's, and tried to have at least 3 in every film. It's the big WOW sequence, or in comedy, the big long laugh sequence, usually a part of that wow is that it's unique "I haven't seen that before". Probably the thing that's going to be in the trailer to make people want to go see the movie. The equivalent of a music exec asking about a new album: "which are the singles?" Having your set piece involve your primary protagonists and arise organically from the story is what can make all the difference. An audience feels the contrivance when a set piece is arbitrarily imposed.
The trench run on the death star, the snake pit in Indianna Jones, the car chase in anything. With Hitchcock's 3 crescendo idea it makes me think that maybe they are appropriately used as the climax to the different acts (assuming you're gonna split act 2 in two which is often the case.)
* now I need to go explore Billy Mernit's Living Romantic Comedy blog, which I pulled up when I searched for Set Piece