Mercy sidles out. Since Proctor's entrance, Abigail has stood as though on tiptoe, absorbing his presence, wide-eyed. He glances at her, then goes to Betty on the bed. Why does Abigail have this response to Proctor?

Abigail loves Proctor and had wanted to get rid of his wife so that she can be together with him. But at the same time, she was afraid of the man for she knows he knows a lot about her more than anyone and that she fears she will be exposed. Her practice of "traffick (ing) with spirits in the forest" could be exposed and she will be put on trial and burned on a stake. So, she was extra careful to not show her real self while at the same time trying to influence Proctor to like her again.

Explanation:

In Arthur Miller's play The Crucible, the main focus of the story is the witch trials of Salem where there are numerous cases of young women involved in the practice of the dark arts. This story is also based off the Salem Witch trials famous in the Massachusetts Bay colony during the late 17th century.

The given passage is from the stage direction in Act I of the play after the girls were seen "dancing like heathen in the forest". The result of this is the 'unnatural' sickness of Betty Parris, the daughter of Reverend Parris. Abigail, the presumed leader of the group in the forest had been involved with the Proctor family until she was discharged because of her affair with Mr. Proctor. And Betty had just exclaimed that Abigail did drink "a charm to kill John Proctor's wife" which is obvious for she loves Proctor and wants him to be hers.

So, when Proctor came to see the sick girl, Abigail instantly felt on guard, for he knew her more than anyone else. And they have had history, and seemingly proctor also knows her hatred for his wife Elizabeth.