In 1984 by George Orwell, when Winston helps the girl with dark hair up, we are told that there is a "frightened" look in her eyes. What do we later find is the reason for that frightened look?
When the woman he recognizes as the dark-haired girl falls:
Her eyes were fixed on his, with an appealing expression that looked more like fear than pain.
The woman, later revealed to be named Julia, has just tripped and fallen in front of him, seemingly landing on her injured right arm, which is in a sling. As she falls, she looks at him with an expression Winston perceives as more of fear than pain.
This expression, for a moment, moves Winston from his normal response of regarding her as an "enemy" he'd like to kill to seeing her as "a human creature, in pain." This is the first sign of Winston's reawakening humanity, which had been almost conditioned out of him by the state. Pages later, when Winston is finally in bed at 11:00 at night ("twenty-three hours") he ponders the situation. He decides that
Obviously she had been frightened out of her wits, as well she might be.
Of course, Winston tends to misread people and project onto them his own feelings, as he does with O'Brien, and as he did earlier with Julia, seeing her as a sexless emblem of the state. In this instance, he assumes she was frightened because of the note she slipped into his hand, which said "I love you." This was a wildly risky and transgressive act in Oceania, and one through which she left herself vulnerable to betrayal.
In Part One, Chapter Nine, of 1984, Winston encounters the girl with dark hair and later reflects that she had looked "frightened out of her wits" at that time. This girl, later introduced as Julia, is not frightened because she is scared of Winston or because she is nervous about declaring her love to him (as she does via a handwritten note). In truth, she is frightened because in the moment before she handed Winston the note, she tripped and fell over, directly in view of a telescreen. As such, there is a very real possibility that Big Brother was watching and that he saw the note in her hand. Winston realises this later:
Whichever way you turned, the telescreen faced you.
As Party members, Winston and Julia are not allowed to have a relationship. In Oceania, all relationships must be sanctioned by the Party and sex for anything other than procreation is not allowed. Both Winston and Julia know that breaking this rule is perilous: if caught, they might be imprisoned and placed in a labour camp or even shot and killed. This explains the terror in Julia's face at the moment she fell down.