Meticulous, Repetitive
Zola's writing style is very detailed—sometimes painfully so. This is partly because he wants to provide realistic descriptions of everyday life. But even more important to his project is the meticulous 'note-taking' quality of his novel.
Again, think science experiment.
When you're conducting an experiment, you have to take a lot of thorough notes of each step. This is what happens in Thérèse Raquin. At each stage of the story, Zola describes in painstaking detail what happens to Thérèse and Laurent, how their temperaments change, how their bodies and minds react to certain situations…
He's like a scientist observing them through a microscope. Which brings us to the repetitive part. Zola repeats himself. A lot.
Again, this mimics a science experiment, where you have to repeat your tests several times to verify that your results are accurate. In Thérèse Raquin, we have constant restagings of Thérèse and Laurent's interactions when they're together in the same room.
At first, everything is hunky-dory, and Thérèse and Laurent indulge all their sexual desires. But as the novel progresses, new variables are introduced: Camille's death, Camille's ghost, Mme Raquin's stroke, and more.
With each of these new variables, we observe all the different "modifications," to use Zola's word, that occur in Thérèse and Laurent's "organisms" (Preface, 10). We read so many scenes of Thérèse and Laurent stuck in their bedroom, unable to arouse their lust, until we start feeling trapped.
This repetitiveness even appears at the level of individual words. Did you notice how often the words "nerves" or "nervous" are used in the novel? This repetition also gestures toward a medical vocabulary of the human nervous system, which works toward Zola's goal of studying "living anatomical specimens" (Preface, 10).
Can we just say "nerves" one more time? Ugh, so tiring. Sorry, we need a nap now.