Throughout the book there are references to chains, cages, tombs, and pits. These contribute to the impression that Laurent and Thérèse are in a state of remorse and are plagued by guilt. The book mentions how they are always clawing at the chains that bound them together. The shop that Thérèse owns is compared to a tomb, where Therese watches corpses walk by in the day.
In his preface to the second edition, Zola writes that he intended to "study temperaments and not characters." To his main characters, he assigns various humors according to Galen's Four Temperaments: Thérèse is melancholic, Laurent is sanguine, Camille is phlegmatic, and Madame is choleric. For Zola, the interactions of these types of personalities could only have the result that plays out in his plot.
Also in his preface, Zola calls both Thérèse and Laurent "human brutes," and the characters are often given animal tendencies. Zola would take up this idea again in his La Bête humaine of 1890.
Similar to the human beast who acts based on instinct, the mechanical man acts like an "unthinking machine."