When I was in high school, we watched in my 10th-grade honors English class, for reasons that now escape my recollection, the Matthew Modine/Nicolas Cage film Birdy. The film is about a troubled youth (Modine) who becomes obsessed with birds, and the idea that he might be able to fly like them. Cage is his friend. At the very end of the film, Modine runs off (I forget why) across the rooftops, with Cage in frightened pursuit, and we, the audience, are made to believe that he might leap off the roof and try to fly. Cage catches up with him, only to see Modine jump! Horrified, he runs over, and we in the audience are expecting a tragic end. But when Cage gets to the edge, he sees that Modine had only jumped to a lower level a few feet down. Modine looks at Cage's horrified/relieved face and says, with all the innocence of a choir boy, "what?" Cut to end credits.
As the credits started in that English class, I turned to my friend and told him that I thought the whole film was one very long set up to that punchline ending. Just one big joke set up. He agreed.
This is also how I feel about There Will Be Blood. We get two and a half hours of gorgeous shooting, deliciously plummy acting, a brilliantly anachronistic score, a very slow-moving plot, and then we get to a scene (which end up being the final scene) in Plainview's home bowling alley. After a ragged, shouty scene between him and the preacher who has served as the antagonist to Plainview throughout, Plainview, for no real reason, beats him to death with a bowling pin. His servant enters, and Plainview,still sitting on the alley floor by the pooling blood of the preacher says (in what actually comes across in a sing-songy punchline cadence), "I'm finished!" Cue end credits.
See? A punchline!