The location for the stairs was a real building located near the area of Churubusco in Mexico City. It was only 30% occupied at the time of the shooting, which is why they allowed the film to be shot on their emergency stairs. Most of the neighbors never even realized a film was shot at their emergency stairs, as they always took the elevator, but the crew used to laugh at how funny it would be if eventually a certain neighbor would want to take the stairs because the elevator was too slow or something like that, and find out an entire crew of 30 persons working there, drawings on the wall and a lot of crazy stuff. Eventually, it did happen, but the people who came in and found everything were not mad. They were, in fact, extremely surprised and happy, taking pictures of everything, as if they were entering a museum, and begging production designer Adelle Achar to never remove any of the art from their stairs.
There's a small Easter egg at the end of "The Similar" (Isaac Ezban's second feature film) that explains the origins of the incidents from "The Incident", and unites the universe and mythology of both films.
On pre-production, the typical casting process was not followed. Director Isaac Ezban choose several actors he had always wanted to work with after seeing them in other Mexican films (he liked Raul Méndez from "Matando Cabos" and "KM 31", Nailea Norvind from "The Other Family" and many theater plays, Hernan Mendoza from "After Lucia", Humberto Busto from "Amores Perros", "Morirse en Domingo" and "Depositarios", and Fernando Alvarez Rebeil from "The Tears"), he even wrote the screenplay thinking about them for their specific roles, and when he needed to cast them, he managed to get their emails from directors that had previously worked with them and he just sent them the screenplay. He was lucky they all liked the original and innovative screenplay and decided to take part on it. For the actors who appear on the second half of the film, a more traditional casting process had to be approached, through agencies and managers, and looking for very specific physical characteristics.
Although a big part of the film takes place in a staircase with only 9 floors, the actual building were filming took place had 17 floors (it was chosen like that on purpose, because of the sequence shot that takes place in the first act), so the crew had to memorize 3 levels or reality regarding the floor numbers: there were the real floor numbers, from the building, there were the "fake" floor numbers (existing only in the universe of the film), and there were the floor number of the day (because sometimes even if the crew was working in, for example, floor number 6 in the film, if they needed to shoot a quick insert from, lets say, floor number 9, they wouldn't all actually go up 3 floors, they just asked production designer Adelle Achar and her crew to replace the number from the wall for that instant), so, it got to be pretty funny how on several times when someone from the crew took the elevator to get to a certain floor, in front of the neighbors that actually lived on the building, and tried to remember which floor they were going, they said things like: "We're going to floor number 6 of the real world, which is number 2 of "The Incident" but number 7 of today". Neighbors just thought then that the people from the crew were crazy or affected by severe drugs.
Shot on four weeks, two weeks locked inside the staircase and two weeks on the road. In the first two weeks, the crew experienced a lot of claustrophobia, as they had to get to the set at 6 am (before sunrise) and leave at 7 or 8 pm (when it was already dark), never seeing daylight. On the second two weeks, it was completely the opposite: all the time in a very open space (a 6 km straight road in Pachuca, Hidalgo, 2 hours drive from Mexico City), on a natural landscape, facing sunburn, rain, wind, mosquitoes and more challenges of nature. They all felt a big contrast while shooting this film, as the audiences are supposed to feel while watching it. Director Isaac Ezban used to ask his crew every day: If you would stay on an incident for many years, would you ratter it happening at the stairs or at the road? When they were at the stairs, they all answered they preferred the road. But when they were on the road, they preferred the stairs.