Trillian, We Hardly Knew Ye
Some movie spoilers through the link.
The new movie, which I otherwise quite enjoyed, engineers a love story between Trillion and Arthur. We get more insights into Trillion's psychology--in a new scene, Zaphod, Ford, and Trillian obtain a "Viewpoint Gun," and we learn that Zaphod is just the latest in a string of men Trillion has dated who are egomaniacal users--but little of her aspirations and intelligence. Never mind that the actor playing Trillian is a bit weak and that the actor playing Arthur manages that English trick of being at once drippy and charismatic. The Trillion of the books has made a series of rational calculations into her adventure; the Trillion of the movie is a space-age Bridget Jones who comes to the sentimental conclusion that Zaphod is just not that into her and that Arthur is the safer, sweeter man.
And here's what killed the film character for me: It is never mentioned that she's an astrophysicist. Narrative economy demands that Trillion be cut down to attainability.