The fact that Gregor is a vermin is a major factor in the way that we perceive and react toward him. Like Gregor and the other, human characters, we are repulsed by the functions of his gross insect body – the brown goo, the creepy-crawly little legs, the gnashing jaws. Yet it's precisely his bugginess that makes him a particularly pathetic character. The other characters seem less human just by virtue of the fact that they don't seem to pity him as much as we the readers do.
Actions speak louder than words here in a story where there's little dialogue. After all, the main character can't really speak intelligibly. Thus the scenes where the characters do interact tend to be almost a pantomime, where gestures and behaviors are more important in communicating how the characters really feel about each other. Reading the story is a lot like watching a silent film, and some of the scenes – particularly with the father chasing around Gregor – are reminiscent of the classic slapstick comedies of Charlie Chaplin or Buster Keaton.
Since there isn't too much dialogue, the story spends a lot of time in the characters' skulls, following their sometimes twisted chains of thought. You could say that the story inhabits the characters' minds too well – it's very easy to lose your critical attitude and get drawn into one or another character's way of looking at things. Thoughts and opinions are also crucial to the way we determine Gregor's status. Because we know how he thinks, feels, and remembers, we know that some residue of humanity remains despite his transformation.