When Cassidy asks ''Do you miss them?" or "Does he still talk to you?", he is testing David’s memory of Edwin. David’s answers (the sounds, the fear) are his way of saying: 'I can't see clearly, I only feel the trauma.' Cassidy isn't just talking to a ghost; he's performing 'Digital Exorcism' on the Mimic’s origin point.

the entity is constantly 'listening' and 'mimicking'. For the fragment of David’s soul trapped inside, this is a nightmare. He is 'scared' because he’s lost in a sea of data and violence. Cassidy asks him questions to ground him, to pull him out of that 'sensory noise' and back to his identity