Gaboriau kept and elaborated many of his predecessors’ techniques; for example, thickening the plot by increasing the tension between two characters with opposite personalities and mentalities. Lecoq’s opposite is his superior in the Sûreté, Gévrol, a man with limited intelligence and without imagination, working faithfully within the framework of police routine, but is jealous and vindictive toward others who are more talented. Contrasted with Gévrol also is the intelligent, inductive Père Tabaret, who directs the investigation in L’affaire Lerouge as an amateur and for his own pleasure. Gaboriau has his passion for detection, saying, as quoted by Père Tabaret:

When reading the memoirs of famous policemen, which rival the most intriguing fables, I was enthusiastic about these men with subtle intuition, smoother than silk, as supple as iron, intelligent and cunning, fertile in unexpected resources, who follow the trail of the crime, the Code in their hands, through the underbrush of legality, just as (James Fenimore) Cooper’s savages follow their enemy in the middle of the American forests. I wanted to be a cogwheel in that admirable machine, to become also Providence on tip-toe, helping in the punishment of crime and the triumph of innocence. I tried, and it’s turned out that I’m not too badly suited for the job.

With Père Tabaret and Lecoq, the policeman is no longer a man working without passion within a dull set of rules, but a man willingly and enthusiastically performing a task he loves.

Two innovations in the detective story which Gaboriau added are the initial focus of the crime and the extended flashback. All of the novels in the Lecoq series begin with a crime, sometimes a murder, sometimes robbery or extortion. The detective investigates the crime in the present; he collects data, interviews witnesses, analyses possibilities and probabilities. He discovers that the crime can only be explained by past events. Then the action switches to an extended flashback elucidating the characters’ past lives. With essential data from the past, the detective returns to solve the crime in the present.

Dorothy Sayers, reviewing Gaboriau in the The Times Literary Supplement, wrote:

The mere reading of the books presents a formidable difficulty, for most of them are out-of-print, and all of them are fat… “I suppose,” says the reader, plaintively, “I shall find him terribly old-fashioneded.” He is old-fashioned; yet he was more so yesterday, while tomorrow may find him back in the fashion.^[2]

Her prophesy, written in 1935, seems to have been fulfilled in the 2000s. He is again published in French and analyzed by Sorbonne professors studying popular literature. Novelist and critic André Gide noted in his Journal on March 4, 1943:

I read without stopping L’affaire Lerouge, Le Dossier 113 and the first volume of Monsieur Lecoq of Gaboriau. I put down the second volume because Gaboriau flounders around in conventional psychology as soon as he leaves his best domain: police investigation, where he shows himself to be an extraordinary pioneer, a forerunner of all detective novels: those of Conan Doyle are only cheap wine compared to his.^[3]

With Monsieur Lecoq, Gaboriau dropped the Lecoq series and created a pair of detective sleuths, a professional policeman, Monsieur Méchinet, and a medical student working to enter the public health services, J. B. Casimir Godeuil. They appeared in a short novel published posthumously in 1878, Le Petit vieux des Batignolles.