Marian Very Different in Book
Remember early in the movie, when Marian says she and Laura are totally different -- and that Laura is so "feminine?" Marian doesn't go on to say that she herself is masculine -- that would make no sense coming from Alexis Smith. But that point is made over and over again in the book. Marian is described there as being swarthy, even "ugly." But she also has the supposedly "masculine" qualities of intelligence and bravery -- she's active, Laura is passive. In solving the mystery, Marian is essential -- she's as much a hero as Walter.
Giving her part to the beautiful and feminine Alexis Smith robs the story of one of its most remarkable elements.