Despite putting the author's name in the title, Mary Shelley's Frankenstein still takes plenty of liberties with the original novel, and even when it's faithful to the letter of the story, it gets the tone all wrong, going for grotesque and overwrought when it should be restrained or romantic. Part of what could have been a series of "classier" and more faithful adaptations of literary horror classics (after 1992's Bram Stoker's Dracula, directed by Francis Ford Coppola), Mary Shelley's Frankenstein instead ended up as a box-office failure, and Coppola (who is a producer here) moved on to other things. The idea of making a faithful adaptation of Shelley's story is still viable, especially since people's perceptions of Frankenstein remain tied up in various pop-culture representations far more than in the book. But even the filmmakers who have Shelley's name zooming toward the viewer at the beginning of their movie can't resist a cry of "It's alive!" or the creation of a bride of Frankenstein.
Much of the screenplay by Frank Darabont and Steph Lady does stick to Shelley's novel, at least at first, and the movie includes the Arctic framing sequence (which is usually the first thing to go) and plenty of time with the extended Frankenstein family. But director Kenneth Branagh (who also plays Victor Frankenstein) turns that family dynamic into something lurid, frequently emphasizing the pseudo-incestuous nature of the relationship between Victor and his foster sister (and eventual fiancee) Elizabeth (Helena Bonham Carter). Branagh overdoes just about everything in the movie, both as a director and as an actor, and while his version of Frankenstein is a bit more soulful and sympathetic than many onscreen portrayals, he still becomes unhinged and aggressive when he's focused on bringing his creation to life, and he doesn't have much consideration for the feelings of people around him.
The creation sequence is a great example of this movie's absurdity, as Branagh does his best to differentiate it from other movie versions, but only makes it more ridiculous in the process. Instead of getting his electric jolt from lightning, this movie's Frankenstein uses ... electric eels! He collects amniotic fluid to immerse his creature in, and then ends up covered in it himself when the creature breaks out of its containment module. There's a certain campy charm to the over-the-top set design of this movie, both in Frankenstein's laboratory and in the improbably cavernous interior of the Frankenstein family estate, which has a giant staircase with no railing that appears to lead to nowhere.
Mostly, though, the histrionic tone is exhausting, and it fails to honor the source material, which seems to have been the main goal of this version. Branagh at this point was known mainly for his acclaimed Shakespeare adaptations, but he takes this as an opportunity to cut loose, and in a way it's a precursor to his more recent work on big Hollywood productions like Thor, Jack Ryan: Shadow Recruit and Cinderella. Those movies are more measured and restrained, though, and Frankenstein kind of gets away from Branagh, especially in the third act, which diverges significantly from the source material to add in a second, even more hyperactive creation sequence and to allow Carter to turn Elizabeth into a horrific bride of Frankenstein monster.
Somehow I've gotten this far without even mentioning Robert De Niro's performance as the monster, and that's partially because he underplays his part while everyone around him is chewing scenery. But it's also partially because his performance is mostly forgettable, even as it sticks more closely to Shelley's vision of the monster and allows him to speak and emote. This isn't De Niro just sleepwalking through the role as he's done so often in his later career, but he doesn't make much of an impression, and his working-class demeanor is not really a good fit for the philosophical version of the monster. Like so much about this movie, his casting is well-intentioned but ultimately misguided, one of many efforts to honor Shelley's original story that only ends up missing the point.