Another element Deep Red takes and expands upon from Four Flies on Grey Velvet is the importance of theatricality. In Four Flies it is highly significant that the key incident which propels the action forward, the protagonist's confrontation and apparent accidental killing of the man who has been following him around and concomitant assumption of his intended role in his persecutors' psychodrama, should take place in a theatre.
Though it is again a visit to the theatre that puts events in motion here, the investigation also sees the others reconstruct the scene for the absent Marc's benefit, a detail which also reminds us of co-investigator Gianna Brezzi's at times curious role in the proceedings. Does she knows more than she is letting on? Here we can also note the moment when Marc awakens having been rescued from the blazing remains of House of the Screaming Child. To simply there would have been simply too mundane and ignominious an end for Argento, Marc having to know the identity of his nemesis, even if “only at the moment of dying” like Frank in Once Upon a Time in the West
As further emerges with the likes of his unofficial (i.e. Opera) and official adaptations of The Phantom of the Opera, the theatre is fundamentally a haunted place in Argento's universe, with this being expressed here by the solidified / sedimented murderous thoughts Helga detects and the way the camera cranes in dramatically on her as if it were an invisible, assaultive presence.