What is the relationship between realism and fantasy in this story? What are some details that make the fantastic story credible?

Obviously is not really credible if we are
talking about any kind of literal or photographic . This is not actually to say that it's
impossible for a story about a man being turned into a giant insect to be plausible or
believable on some level, as the David Cronenberg film The Fly
demonstrates. Because the plot of The Fly is invested with at
least a veneer of science, an explanation of sorts of what has happened, and realistic reactions
from the characters about the transformation, it is credible in its way. In Kafka's story, these
elements of realism are lacking. There is no attempt to explain how or why Gregor has suddenly
become an insect. His family reacts with horror, but they seem frozen in their responsethough
such behavior is perhaps realistic in the sense that an event like this is so horrible and
unprecedented that people would, in fact, be paralyzed by it. However, the family's reaction
seems to be that what's happened is more of a shameful embarrassment...