In this scene, Nicolai Rostov is struggling with the shift between seeing the French as "the enemy" whereas Boris is already there;. Boris is a political animal and has sensed the shift of political winds. Tolstoy's characters are often divided into two groups: those who are conventional and those who are non-conventional. He does not respect "conventional" people.
"The expression of annoyance had already disappeared from Boris's face; evidently having reflected and decided what to do, he took him [Rostov] by both hands with a particular calm and led him to the neighboring room. Boris's eyes, looking calmly and firmly at Rostov, were veiled as if by something, as if some sort of screen -- the blue spectacles of convention -- had been put on them. So it seemed to Rostov." -- Lola
from page 515-516 of original text
page 408-410 of Pevear/Volokhonsky translation