In Mary Shelley's Frankenstein, each of the main characters, Walton,Victor Frankenstein, and the creature, is consumed by an idee fixe. That is, they are obsessed with one idea.
With Walton, it is the exploration of the North Pole that is his idee fixe. For, it is not until the crew beg him to turn back that he does. Still, like Victor, he listens to the creature's poignant tale and lets him live.
With Victor Frankenstein, his "vaulting ambition" and pride that drives him to "play God" and create his own being. Then, after he has given life to an aberration, he is repulsed and flees. After he learns of the death of his brother William, Victor's ego will not permit his being honest and claiming responsibility for the deaths of two people. Instead, he vows to pursue the "monster" he has created, allowing him to kill more people in his path of revenge against Victor. Finally, the creature becomes obsessed with avenging himself against Victor after he is rejected by the DeLaceys. He blames Victor for giving him life and making him hideous to others; when Victor reneges on his promise to make a mate, the creature continues in his obsession by killing Elizabeth on her wedding night when Victor leaves the room.
Each of these characters were living a life that is a result of their inner obsessions, desires, and hopes. However, these same elements have left them isolated from the rest of humanity.
Walton is a hardcore explorer which had not had much contact with the world until he accidentally met Victor during his and the monster's exile to the Antarctic.
Victor was a hardcore naturalist obsessed with the processes of life and creation. In his arrogance and obsession he created the monster out of dead body parts only to become a slave to its whims, and a prisoner of his own conscience.
The monster had no choice but to escape the society which shunned him. He had no options, but to be a monster, a creature of heidious beginnings and uncertain ends.
This is how each of the characters were prisoners of themselves.