To Zola, scientific writing meant fiction whose facts were thoroughly researched. Science was the new fashion. Balzac, Flaubert, and many other writers in France, Italy, and other European countries were also experimenting with a scientific method of fiction writing. Zola's particular use of science, aside from the thoroughness of his research, may be seen in his study and application of the latest medical knowledge. In Zola, when a character is stricken with an ailment, the author knows of what he is writing.

Zola researched his books much as though he were a journalist writing nonfiction. In the 1880s, when he began Germinal, he lived with miners, drank with them at night, and went down into the mines; later, for La Bête humaine, he traveled on a train dressed as an engineer. In 1872, he lived in the belly of Paris. He spent his nights at the Courbevoie Bridge, where wagons loaded with food came into Paris from the west. He would scramble alongside the horses from the customs gate at the edge of the city eight miles to Les Halles with his pencil at the ready. He spent endless hours taking notes in the market and also examining the view from different approaches. A friend of painters—Cézanne was his closest boyhood friend—he tried to use words like brushstrokes. His visual descriptions can be labored. Several of his descriptions of food are so lengthy they will try the patience of all but the most dedicated foodies, though these remarkable passages are occasionally worth it, such as the depiction of Roquefort cheese resembling the face of an aristocrat stricken by a disease that attacked the rich who ate too many truffles. On the other hand, what a rare opportunity to view nineteenth-century French food firsthand.

His characterization of the bombastic painter Claude Lantier must have been shaped by years of counseling the chronically dissatisfied Cézanne, the impassioned perfectionist who once worked on a portrait of Zola and then, to the author's outrage, destroyed it because he did not like the way it was turning out. Cézanne was constantly flying into rages and depressions and tearing up his own work. Certainly Zola must have recalled his outings with Cézanne in Provence when he wrote about Florent and Claude in the countryside enjoying a long hike together. Claude Lantier has physical similarities to Cézanne, and he dresses like him in a red sash, felt hat, and old overcoat. But many of his views on art and his enthusiasm for morning markets are purely Zola. Novelists get into trouble when borrowing parts of friends for characters, and the recurring character of Claude Lantier strained Zola's friendship with Cézanne, who finally, after the 1886 publication of the novel L'Œuvre, about Lantier and the bohemian world of painters, stopped speaking to his childhood friend.

Zola also hounded the police for information on the administration of the market. This scientific approach, however, did not spare the speedy writer from the kinds of little errors in French grammar and spelling that a more careful editor might have caught and are not passed on in translation.

The beginning of the Rougon-Macquart cycle was delayed by the Franco-Prussian War, the German siege of Paris that forced Zola and many Parisians into exile in the provinces, and finally the overthrow of Napoleon III. To Zola and many French, the Bonapartes had been France's recurring curse, “a strange family that won't die,” he wrote, “that persists through its pale and moribund offspring.”

These events helped Zola's master project. By delaying the actual writing, he had time to plan out the novels, which established an orderly method so that when he did start to write each morning, he would consult the notes he had prepared, which he kept on index cards. His progress was so steady and programmed that he did only one draft, handing it in with only occasional cross-outs.

But the events also closed the book on the Second Empire and allowed Zola to write about a finite period of time. Newer events could not overtake his novel. In 1871, Zola wrote that the novels could now be set “inside a closed circle; it becomes the tableau of a dead reign,” of “a strange era fraught with madness and shame.”

He begins his fictional family, the ancestors of Beautiful Lisa and her nephew Claude Lantier in the The Belly of Paris, before the revolution that would unleash all the political forces of Zola's time, in Provence, just outside Plassans, the fictional name of his native Aix-en-Provence. Adélaïde Fouque is the daughter of landowning peasants outside Plassans. When she is eighteen her father goes mad and dies, and, left alone, she marries an illiterate gardener named Rougon. Three months after their son, Pierre, is born, Rougon dies. Adélaïde takes up with her neighbor, a brutal, alcoholic smuggler named Macquart, who lives apart from her in a windowless shack but nevertheless sires two illegitimate sons with her, the first Rougon-Macquarts.

The family appears in Le Ventre de Paris and winds its way through the cycle of novels. Lisa's sister Gervaise, the mother of Claude Lantier, is a principal character in L'Assommoir La Terre is the story of their brother Jean Macquart.