Shelley uses nature in particular to make the story scary. Whether it is a storm, or lightening during the creation of a monster, the weather creates moments of great fear. The locations of the Arctic with its great cold and ability to freeze and snap as well as the jagged Swiss alps and dark forests make us as readers unsure and uneasy about what is going to happen to characters.
Shelley uses foreshadowing to build suspense and keep us wondering what is going to happen with characters. For example, in the beginning, we learn that Victor Frankenstein was chasing a hulking brute of a man. Yet, we do not learn the identity of that man until much later. We have evidence of murders that later prove the murderer to be the monster of Frankenstein.
Shelley uses character to portray themes of life that have concerned her. She feared herself not being about to provide for her own children. Thus, the over-nurturing character in Elizabeth is difficult to watch as she cannot save William or Justine. The intellect we see in Frankenstein's creature makes us want to like him and trust him. But as time goes on, that intellect reveals reason and purpose for why he commits the crimes that he does. This character demonstates that we often create our own problems and eventually fears.
Concerning Frankenstein being a horror story, I always chuckle a little when I read Mary Shelley's account of how she thought of the idea for the story, and how she wanted to create something so horrific. By our standards today, the novel isn't really what we think of as scary. It certainly doesn't belong in the category of today's slasher movies.
Her novel is mostly one that involves horrific ideas. Humans creating life is creepy, and a bit horrific. And creating life in secret while obsessing and suffering from what one might call fits is a bit scary. Irresponsible science is scary (just look at the twentieth century's creation of new weapons). But this is a kind of somewhat sophisticated horror, a horror of ideas.
Shelley does use some traditional methods to make her novel scary, however.
- Blood and gore. We are certainly familiar with that being used in stories today. Body parts abound, scars, the hideous appearance of the creature, the destruction of the female creature that is half finished.
- The killing of the innocent. The creature kills a little boy.
- Plot twist. The reader's focus is on Victor and the creature killing him on his wedding night, but the creature kills his wife instead.
Though the novel might not be so scary by our standards, it has actually become scarier over time. The creation of life was a fantastical dream at the time it was written. Today, cloning, though not actually the creation of life, is certainly creepy in the same way the novel is.