In her insightful analysis of Four Flies on Grey Velvet in relation to the Deleuzean time-image, Colette Balmain emphasises the way in which the film foregrounds the Nietzschean "powers of the false".
Though Nina Tobias knows full well that Roberto is not her father and only looks like him, she nevertheless successfully manages to entertain this apparently life-enhancing delusion and successfully knowingly (dis)believe to the point of positioning Roberto as her father in order that she can extract her revenge; I say 'apparently' because Nina's tragedy is that she has not been able to overcome her past, which has determined her present and future, culminating in her demise at the climax of the film. At this stage in his career, Argento, that is, is perhaps not quite completely willing to endorse the murder of innocents, no matter how life-enhancing it may be for the other.
In passing, we might also wonder if this is also something of a reason why the anti-Nietzschean Hitchcock could never quite be a modern, time-image film-maker, that he was still too wedded to a pre-Nietzschean morality and absolutist world-view to allow the killers of Rope, Stage Fright or Strangers on a Train to get away with their otherwise artful crimes. (Yes, Hitchcock would sometimes cite De Quincey's On Murder as One of the Fine Arts, but did he really believe it or was it just for shock?)
Nina's demise itself is replete with meaning insofar as it recapitulates Roberto's decapitation nightmare, thus again presenting the actualisation of the virtual, and is tellingly immediately precipitated by Nina's looking back rather than forwards and thereby failing to see the truck parked in front of her.
In a similar way, though Roberto has not in fact killed anyone, his belief that he has is sufficient to position him within his appointed role in Nina's psycho-drama. Equally, however, this selfsame fact serves to also highlight one of the ways in which Four Flies on Grey Velvet functions as more of a movement-image film in other regards.
Nina, that is, can rely on Roberto's automatic sensory-motor response to the stimuli of having found himself been followed and now getting the opportunity to confront his persecutor, to put her scheme into motion.
This said, the importance of the virtual and the actual here is further confirmed by the location of this key scene in the film, namely an empty theatre. Argento also foregrounds Roberto's entry into this space through a hyper-realist series of four successive shots of his passing through bright red curtains. The first two of these and the fourth are framed from Roberto's point-of-view, but the third is from the reverse angle and thus presents Roberto's entry more from the (impossible) point of view of someone on the other side of the curtains, already positioned within the theatre.
Again, however, if this perhaps represents the actualisation of the virtual, as something Nina had previously planned for 'in her minds' eye', the fact remains that Roberto does not remain external to the set, as the passive seer of the time-image would, but rather changes the relationship amongst the other elements through his active presence amongst them in the manner of a movement-image protagonist.
In this regard, the sequence also presents a different working through of the analogous moment in The Bird with the Crystal Plumage where Sam's inability to fully extend his sensory-motor response into a completed action and his corresponding becoming seer are pivotal to all that follows thereafter.
In a similar vein, Profondo rosso will present a third variation on this scenario, as Marc enters into the set and the action without hinderance, but fails through his perceptual conditioning to notice the vital detail to which he should have responded until it is too late. The image which could or should have proved an impulse fails, that is, to do so.
If, then, Four Flies on Grey Velvet presents the actualisation of one virtual possibility, the one Nina had anticipated through her awareness of her husband's predictable - masculine - response to the stimuli presented before him, palimpsestically The Bird with the Crystal Plumage and Profondo rosso present alternative incompossible versions of not dissimilar scenarios.