James is much more complex, dry, cold, dark, and painful. Fiona Finlay, producer of The American, agrees with Holland's assessment. I'm not sure Henry James believes that. If you want to be true to the spirit of the book, that is true to the spirit of the book. No matter how loyal the filmmakers' intentions, James cannot be transplanted intact from novel to film.
Decisions have to be made.
While there is room to digress in literature, in the theater stories must maintain their momentum. Some characters do have to drop off the table. I mean, I have 90 minutes or something. The reader has hours. The new film adaptation of The American contains several striking changes from the book, the most obvious being the film's opening sequence.