He'd just completed his "animal" trilogy of gialli ( The Bird with the Crystal Plumage , The Cat O' Nine Tails , Four Flies on Grey Velvet) and nursed a belief that the genre had, if not run its course entirely, at least run its course for him.
The elements of Argento's “animal trilogy” ( The Bird with the Crystal Plumage , Cat O' Nine Tails and Four Flies on Grey Velvet ) don't particularly lend themselves to explanations within any system of associations implied by the rest of the filmic experience.
Though one of Argento’s lesser known works, in part because Paramount chose for decades not to allow its release for the home market, Four Flies on Grey Velvet is in fact a key film in the giallo king’s oeuvre, representing a crucial bridge between his earlier, more conventional mystery thrillers and his subsequent explorations of the oneiric, the outrageous and the irrational (in films like Deep Red, Suspiria and Phenomena ).
When it comes to suspense and blood and gore, there is plenty implied, but the moments of horror are not quite as striking as they are in, say, Deep Red or Tenebre (Four Flies on Grey Velvet shares key plot points with both of those later Argento gialli) — however, there is some good creep-factor in the use of masks and puppetry, shiny, sharp blades, and chortling over-the-phone taunts.
There are elements here that work extremely well including the camera work and overall atmosphere that Argento would use to far greater effect just a few years later in Deep Red .