Brandon in the novel is only just younger than Marianne's mother (in fact, Mrs Dashwood would have been a more obviously suitable match for him in many ways), and depicted as wearing a flannel waistcoat and complaining of rheumatism. He's dull. So is Edward Ferrars, whom Elinor marries -- a stolid, decent young man without an original or passionate bone in his body.
The genius of the Ang Lee film is in the casting of Alan 'Sexy Older Man' Rickman, and depicting him as crossed in love and smouldering under his staid surface, and in casting Hugh 'Stammering Charmer' Grant as Edward, and reinventing the character as not so much dull as diffident, but also funny, kind and deep-feeling -- in fact, the classic stuttering Hugh Grant poshboy.
The film in fact completely reinvents the novel, where Austen's message is that while unromantic, pragmatic, buttoned-up Elinor is perfect as she is, silly, romantic wild-child Marianne is utterly wrong, must be punished (and almost killed) by the plot for her unguarded behaviour with Willoughby, and only gets the consolation prize of Colonel 'flannel waistcoat' Brandon. If you read the end of the novel, you'll see that even Austen can't be bothered making it sound as if Marianne ever feels for him the way she did for Willoughby -- she's a sadder and wiser girl, and her transfer of affections from Willoughby to Brandon is perfunctorily described.
If the screenplay and casting hadn't flipped that (to make it seem that Elinor is at fault for being too staid too, rather than being utterly correct in her behaviour throughout, and to make us sympathise with Marianne's teenage romanticism, and to give them both appealing matches), it would never have been such a big hit with 1990s audiences. 'Be sensible and guarded, or you'll almost die and end up married to a dull father figure' isn't exactly what modern audiences tend to want in romantic films!