Compressing a novel into a two-hour (or often less) film is difficult.
Changes are made not only to fit a sitting audience, but to satisfy (in the '40s-'50s) censors, as noted in another thread, and to fit the characteristics of the cast members who have been selected to play the parts.

Films from books have seldom satisfied me; plot development is chopped and sometimes not intelligible unless one had read the book previously.
Two from the 1940s; "Green for Danger" was altered to fit the detective's character, and "Dimitri" to fit a different sort of person caught up in a mystery, but were fairly successful adaptations.