Why does Julia find it quite plausible that Winston trusts O'Brien on the basis of a look in the party members eye?

Unlike Winston, who thought long and hard about his actions and the people around them, Julia formulated her opinions instinctually.... without thought.

She was used to judging people by their faces, and it seemed natural to her that Winston should believe O'Brien to be trustworthy on the strength of a single flash of the eyes.