Most writers might describe characters' thoughts a lot during the slow points of a plot, but when the action heats up, they cut some of the long descriptions and speed things along.
Not James, though, hoo-boy. Peep this passage:
[Merton's] presence, his face, his voice, the old rooms themselves, so meager yet so charged, where Kate had admirably been to him—these things counted for her, now she had them, as the help she had been wanting: so that they only stood there taking them all in. (18.104.22.168)
Now just so we're clear, this is a climactic scene in the book. It comes right after Susan has told Merton that Milly is dying. But James never pushes the gas pedal too hard. He continues to describe absolutely everything in the same level of detail as he did in the opening paragraphs of the book. James is as much a psychologist as he is a writer. A reader who picks up a Henry James novel needs to be more interested in people's thought processes than plot, because thought processes is what makes a Henry James novel so Jamesian.
We're not saying that his plots are shabby—The Wings of The Dove has an awesome plot full of intrigue and deception—but the best stuff in the novel is the characterizing detail.