For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
After White's failed suicide attempt, Black and White get their debate started with few preliminaries. They quickly sense each other's basic positions: the non-belief and anti-life views of White, and the optimistic Christian perspective of Black.
Act II
Punctuated by Black's speeches in favor of everlasting life and seeing the basic divine essence of everyone, this section comprises the meat of the debate. Black is at his most active and creative in trying to explain his worldview. He gets White on the defensive at points, and doesn't lose hope.
Act III
This is when things unravel. After eating, White resists any talk about God or the meaningfulness of existence, and instead he preaches his own intense desire for death and despair, advocating for the complete spiritual emptiness of life on earth. His refusal to enter into the debate on Black's terms throws a wrench into the proceedings. White walks away to (probably) kill himself, while Black breaks down before reaffirming his faith in God.