"Stop calling it her!"
Stevie erupts when Martin continues to call the goat that he is in love with a "her." It's beyond her understanding of how he can be in love with an animal and even reference the creature in a way that is so familiar.
"Shut up; so long as we don't screw up. And you've screwed up!"
Stevie has just told Martin that they are not like other people they understand the deeper things of life, and as long as they don't screw up they will keep a straight line to death. Martin attempts a response, but Stevie cuts him off with this line.
"What have you done!? Oh, my God, what have you done!?"
Martin says this after seeing Stevie come into their home with his lover, the goat; she has slit the goat's throat, killing it.
