I haven't read the book, but a number of reactions have essentially said that the movie is more of a companion piece to the book than a straight adaptation. As in, it's the same, basic story, but where the book has [X] as the main plot, the movie has [Y] as the main plot. I may not have this 100% right, but apparently it was initially more of a straight adaptation, and DiCaprio was originally meant to play the Plemons character. But he was ultimately drawn more to the character he ended up playing, and all I know for sure is that the script was then rewritten numerous times to accommodate the change. That said, apparently Plemons' role is still quite significant.