Personally, the moment in Magritte's response to Fantômas that most interested me was at the end of the coup de theatre when he posited that Fantômas can only be captured by entering his dreams.
His notes were published in 1928, the year after he moved to France and became acquainted with André Breton and joined the surrealist group. André Breton defined the goal of the surrealists as, 'to resolve the previously contradictory conditions of dream and reality into an absolute reality, a super-reality'.
Within the context of this definition, one can imagine that Magritte's work, The Menaced Assassin, mentioned in the preface to the Notes Sur Fantômas, depicts Fantômas' version of this so-called 'super-reality'. The painting clearly depicts a dream-like place. Because of Fantômas' casual demeanor, listening to records at a crime-scene, I imagine that it's the dream of Fantômas himself, infiltrated by bowler-hatted assailants as Juve imagines in Magritte's coup de theatre. Trouble is, Fantômas is one step ahead of his pursuer, even in dreams.