Branagh doesn't stick to everything in Shelley's book. The whole episode of the creation of the female, for example, is developed differently. In the film, Elizabeth, Victor's wife, is killed by the monster, and Victor tries to resurrect her for himself. Then, the monster challenges him for the creation. The resurrected Elizabeth then kills herself in disgust at what she has now become. In the book, the monster kills Elizabeth as revenge after Frankenstein destroys the female he is creating to be the monster's consort.
Also, Branagh's elaborate scenario for the making of the monster owes a great deal more to James Whale than it does to Mary Shelley. Mary Shelley has very little to say about how the monster was made and life created, possibly because she hadn't the slightest idea how to present a convincing explanation. All the thunder and lightning and electricity that animates Branagh's creation scene comes right out of the Whale tradition. He adds a touch of acupuncture to put his own signature on it, but whatever it is, it's not Mary Shelley.
Of course, it is wrong to quibble over the fact that the movie differs from the novel, even though coming closer to the novel is obviously one of its aims. After all a movie is not a novel. It couldn't possible do everything the novel does. Besides, it does include much of the novel's frame as Frankenstein tells his story to Robert Walton (Aidan Quinn), the obsessed explorer who is a foil character to the scientist in the book. It does include the whole episode of Justine and the death of Frankenstein's little brother. It even makes sure that Frankenstein has the right first name. How much can one ask for?
The real problem with the film is the acting. Most of the actors chew the scenery without mercy. Branagh himself is the main offender. His performance is pure camp; there isn't a melodramatic string he isn't willing to pull. And the rest of the cast takes its cue from him. In fact the only major performance that doesn't go over the top is the one where it would be most justified: De Niro's monster. More often than not, De Niro plays in a minor key. His monster is almost subdued, especially set against the turmoil of Branagh and Bonham Carter. John Cleese in the role of Professor Waldman, Frankenstein's mentor, is, like De Niro, somewhat less melodramatic.
If Mary Shelley's 1818 novel needed another adaptation to the screen, this wasn't it.