In the novel, Julia's marriage to Rex Mottram is in many ways an act of rebellion, as it happens over her family's objections. In the movie, the family want her to marry Rex, and agreeing to do so makes Julia seem much more weak and dominated.
In the novel, Jasper warns Charles off of Anglo-Catholics* Anglicans with Catholic sympathies in general, saying "they're all sodomites with unpleasant accents". In the movie, Jasper remarks, "Sodomites, all of them!", but is speaking specifically of the Roman Catholic Sebastian and Anthony Blanche, turning a piece of rhetorical bigotry into a specific comment on the characters' sexuality.
Dyeing for Your Art: Averted. Hayley Atwell was asked to lose weight for the role of Julia (who, to be fair, is described in the novel as fashionably waifish) but Emma Thompsonput her foot down upon finding out, threatening to leave the production unless Atwell was allowed to maintain her natural weight.