When and how does nature play its role in Frankenstein by Mary Shelley?
While Mary Shelley writes a good deal about human nature in Frankenstein, this question seems to indicate physical nature as opposed to human nature. Yet one way to answer this might be to say that Shelley develops the role of human nature when she is exposing her beliefs about humanity, both the dark side and the noble sides, by describing or illustrating how human nature yields the benefits or perplexities of life. In a similar vein, Shelley develops the role of physical nature when she describes the beauties and the dangers of nature to develop how nature acts as a metaphor that parallels her discussion of human nature.
It was the secrets of heaven and earth that I desired to learn; and whether it was the outward substance of things or the inner spirit of nature and the mysterious soul of man that occupied me, still my inquiries were directed to the metaphysical, or in its highest sense, the physical secrets of the world.
Meanwhile Clerval occupied himself, so to speak, with the moral relations of things. The busy stage of life, the virtues of heroes, and the actions of men were his theme;
To illustrate the latter part of the above answer, which the question seems to indicate, let's examine one or two instances in which Shelley discloses her theory of physical nature. Shelley early on ties the beauties of nature to the inner soul of Frankenstein. The letter-writer says that Frankenstein leads a "double existence" in which, on the one hand, he is "overwhelmed by disappointments," yet, on the other hand, he is enriched by the "beauties of nature" that he finds "elevating his soul from earth." Shelley here equates nature's beauties, like the "starry sky" and "the sea," with the highest movements and impulses of human's inner nature by using juxtaposition of the two concepts and analogy between the two existing states.
In addition, Shelley creates a dramatic contrast between the "tamer scenes of nature" and the "wild and mysterious regions" of Frankenstein's narrative of his misfortunes and woes. The contrasts of these opposing parts of nature are analogous to the duality of Frankenstein's life and experience: first he is a tame researcher and scholar, next a "wild and mysterious" pursuer of the deepest mysteries of life.
Were we among the tamer scenes OF nature I might fear to encounter your unbelief, ... but many things will appear possible in these wild and mysterious regions