Kenneth Branagh's 1994 filmatic adaptation of Mary Shelley's Romantic novel Frankenstein contains many elements not seen in the original novel. While both the novel and the film open in similar ways (with Walton's expedition and the coming of Victor), the movie does tend to allot the women of the film more power than the novel does.
For example, Victor's reaction to his mother's death is far more emotionally harrowing in the movie than the text. Shelley's "version" shows Elizabeth to be far more affected by Caroline's death than Victor. Victor's obsession with the women in his life is far more evident in the film.
Victor's obsession in the novel is the reanimation of life and the destruction of the monster--far different from the film. Instead of the lightning triggering Victor's interest in reanimation and science, his mother's death exists as the trigger. Following his victory at creating life, Victor feels that anyone around him who dies (and he cares about) needs to be brought back--shown by his bringing back of Elizabeth in Branagh's version.
That said, not many filmmakers create exact replications of the novels they chose to rewrite for film. Many reasons explain the existence of the differences between an original text and an adaptation.
First, some novels are too long for filmmakers to produce in completion. Given limitations regarding budget and time frames, some aspects of the original text must be omitted.
Second, many people are involved in making a film. A director must choose the right people to cast, the correct costuming, and the appropriate setting. With so many things to create visually, some things may be left out of the adaptation based upon preferences of those involved in the project.
Lastly, when creating a filmatic adaptation of an original, the writer/producer in charge focuses upon the things they wish to bring to the forefront of the film. Essentially, this is much like many people reading the same work will come to the conclusion about the different levels of importance regarding different parts of the text.
Branagh seems to wish to highlight the relationship between Elizabeth and Victor far more than Victor and his creature. Given that love has always been a theme important to moviegoers, Branagh brought the love between Victor and Elizabeth to the front of the novel (instead of leaving it hidden as Shelley did).