The scene where Philippe falls over with the chair wasn't scripted. The swing was so powerful that the actor who was tied into the chair lost his balance. It caused quite a scare among the crew as a real knife used for closeups fell close to the actors body.
This cut ended up in the final version of the film.
When shooting the outdoor scenes by the lake the temperatures were as low as minus 12 degrees Celsius. When Philippe and Paul are having the conversation by the lake, you can see the actors lips are purple from the cold.
Introspectum Motel was shot in 17 days.