I've just been re-reading a useful little introduction to the giallo by John Martin, one of my favourite writers on the subject and Italian exploitation cinemas in general.
While he goes on in the piece – published in The Dark Side, of all places – to suggest his own quasi-definition, that "The essence of the giallo resides in two factors: style and savagery,” it's the introductory remarks by a number of other luminaries that I find really thought-provoking.
“That's a tricky one. I guess the first thing that springs into my mind is people with big black gloves, with zippers up the sides, and big knives, to stick into people... but its more complex than that...”
– Richard Stanley, offering a definition based around visual iconography and immediately acknowledging its shortcomings.
“It defines a genre of mysteries in which the discovery of the criminal's identity is less important than discovering how the crime was done.”
– Maitland McDonagh, emphasising narrative and thematic elements but raising questions over whose discovery of how the crime was done that matters.
“You mean Argento? Bava? Those films are blood operas...”
– John McNaughton, picking out defining auteur figures and a national cultural tradition.