Mary Shelley included a stanza from Coleridge's "The Rime of the Ancient Mariner" in chapter five of her novel Frankenstein. This inclusion is found after Victor has found success in re-animating life and fled from his loft (given his fear of the Creature).
This is the excerpt of the poem Shelley includes:
“Like one who, on a lonely road,
Doth walk in fear and dread,
And, having once turned round, walks on,
And turns no more his head;
Because he knows a frightful fiend
Doth close behind him tread.”
Given his fear of the Creature, Victor (while walking the streets of Ingolstadt) contemplates upon the lines of the poem. The poems exhibits his own feelings regarding his self-isolation and the fear he possesses regarding the Creature following him.
"Like one who, on a lonely road / Doth walk in fear and dread"
Victor is out, alone, late at night. He is trying to escape from the Creature he left in his flat (who is fears and dreads will find him).
"And, having once turned round, walks on / And turns no more his head"
This allows readers to create a mental image of Victor walking along the road at night, turning his head once, and deciding that if he turns again he will (or may) see the Creature. His growing fears force him to no longer look at what could be following him.
"Because he knows a frightful fiend / Doth close behind him tread"
Compounding the two previous lines, the poem illustrates why Victor refuses to turn around to see if he is being followed. His fear of being followed is too strong to be solidified by his turning and actually seeing the Creature.
Essentially, the poem speaks to the great fear Victor possesses regarding the possibility of the Creature following him. Given the prior allusions to the poem (Walton speaking of the mariner in his opening letters), the insertion of the poem here not only proves appropriate given Victor's fears, it also illustrates the doppelganger effect (as Walton and Victor being parts or mirror images of each other).