Apparently Scarlett's character's name is Laura, but I don't recall it ever being muttered, but the end credits say so. Laura is not from Earth, and her job is to lure men to a rundown abandoned house so their innards can be harvested. And when I say innards, I mean all that is left is their skins, like a molted reptile.
This is the first "under the skin," and it is very literal. The aliens value all that is under the skin of humans. I assume not just men, it's just easier to lure men than women.
The more Laura is among humans, the more outer differences she observes and then the more inner difference she becomes aware of. She begins to see humans for more than something to be harvested.
Her change in attitude culminates when she encounters a disfigured man. Many humans would find him revolting or make fun of him or simply shun him. Accepting his disfigured face (or perhaps not even recognizing it as disfigured) allows Laura to appreciate what is figuratively "under the skin" of humans. She does lure him to the alien trap, but then sets him free. And Laura begins to wander, no longer harvesting humans.
Laura may be adept at luring men to their doom, but she does not understand how to live amongst humans. A good Samaritan helps her out. However, she offers no conversation or details about herself and thus he cannot see "under her skin." He is polite and helpful but still tries to have sex with her. But she does not understand sex. She is almost robot-like as he attempts to seduce her, but not only is she mentally and emotionally incapable of sex, also physically. It goes nowhere and she leaves.
Laura makes her way to a secluded wood, where walking a path she encounters a man who gives her tips on the area. She falls asleep in a shelter and the man shows up and begins to assault her. She escapes into the wood. He catches her and attempts to rape her. But then her human body suit rips and he can see part of the alien beneath. He douses her with petrol and sets her on fire.
Story progression: Aliens value what is the under the skin of humans and kill them. One alien discovers what is truly under the skin of humans and can no longer kill them. Men see only a beautiful woman and care not what is under the skin, they just want to have sex with her. A man discovers the alien under the skin and kills the alien.
The skin, our outer selves, this is the packaging. And it is important. Your packaging reveals your lifestyle, it reflects the decisions, choices and events in your life. What is under the skin, this is also important. It reflects your motivations, and the thoughts and feelings you have about the decisions, choices, and events in your life.