Content Warning: The source text references multiple deaths by suicide and deals with the psychological effects of trauma, loss, and grief. It also uses stigmatizing and potentially offensive language to refer to people experiencing mental illness.

Lock Every Door explores the Psychological Effects of Isolation and Loneliness via protagonist Jules Larsen’s experiences at the Bartholomew. Jules takes the Bartholomew apartment-sitting job because she doesn’t have a family, partner, home, or job. Therefore, Jules is in an alienated position at the outset of the novel. Jules’s eagerness for companionship leads her to trust the Bartholomew’s residents and dismiss Chloe’s concerns. However, when Jules starts probing into the Bartholomew’s mysterious history, she begins to realize how isolated she truly is. The Bartholomew and its residents prey upon isolated and lonely individuals like Jules. Before Jules agrees to take the Bartholomew job, Leslie interviews her, probing into her social connections to ensure that she does not have a support system. Further, in Chapter 30, Jules and Dylan realize that none of the apartment sitters have family. Through this, Riley Sager illustrates the ways in which isolation opens people up to manipulation and other vulnerabilities. The building’s rules against associating with permanent residents further isolates the apartment sitters, effectively estranging them from society.