I remember when I watched Parasite that when Mun-Kwang, the housekeeper who had been fired, shows up at the end of the family’s drunken celebration in the living room that this marked the Transition in the story. The Drama now turns into a Thriller with the revelation of Kun-Sae, Mun-Kwang’s husband who has been hiding out in the basement.
I checked my watch in the theater and noted it was around the 60+ minute mark: precisely midway through the film.
In the script, it’s 73 which is almost precisely midway through the script (140 pages).
Thus the big shift in genre and tone happens at the halfway point, and there’s a certain kind of beauty to that from a screenwriting standpoint, such a profound plotline point happening right at the midpoint of the story.