There are many instances of the number 9 throughout the film, in reference to the main plot device being the drug Cloud IX. For example, Carolus Guess's shoe size can be seen on the shoe box as size 9, there is a 9 of Spades playing card seen in the intro, the dead girl wears a Boston Celtics jersey with #9 on it, the Duchess's residence is on the ninth floor, etc.
There are various callbacks to jokes from the first film. For instance, Carolus hallucinates Alphonse holding a tray of bananas - bananas were a prevalent visual joke in the first film. Another reference is in the dead girl's house when Carolus constantly leaves and returns to the victim's body while trying to connect the dots of the mystery, just like in the joke in the final act of the first film when he constantly leaves and returns to Alphonse's body while connecting the dots of that mystery. The scene in the barn with Alphonse has several callbacks to the first film - "Don't lie to me Alphonse!"
Alphonse wears a black shirt in this film (as opposed to his white shirt in the first film) to symbolize that he is dead and passed on from this world. This actually happened because the cast & crew had no white tank tops when Connor Lemcke was first available to film.
The shot of the Duchess turning her head to Carolus when he asks if her husband has died took over 20 takes.
There was intended to be as many as five or six shots done with drone cinematography in the film. However, only two were shot, and only one made it into the film - the establishing shot of the parking garage. The other was only used in trailers - Carolus alone on top of the empty parking garage. The other planned shots could not be done because the drone hit a tree and broke while shooting the establishing shots.