There Will Be Blood Mark'S REVIEW
Evil is as evil does
The key moments in the movie are all unassuming. For example, Plainview knows that he will have to confront a major character well before we see him act openly. We can see the undercurrent well before the fight but it is not until the end that we find out why. Likewise with his long lost brother. Everything turns on small, easily missed pivots.
That is the power of the intrusive and brutal soundtrack. It often seems to peak when not much is happening but that is because of what lies beneath the action or lack of it.
It's not really based on the book except as a milieu for a new version of Plainview to emerge.