Concerning Better Watch Out by Chris Peckover, regarding Luke's arrangement with a pen in front of the door of his room at the end of the movie. What was the purpose of it?
Correction: It was a pencil not a pen.
At the starting of the movie we see that Luke's mother calls Ashley (the babysitter) and tells her that he sleep walks. So tell her to give him sleeping pills and after that close the door and put a pencil on the handle, and if he walks around, it can assume that from the pencil.
And at the end of the movie Luke needs to put the pencil on the door, so his parents would think that he was sleeping all the time.