Question

Hawthorn uses an especially expressive simile to describe the emotions that momentarily reveal themselves on the face of the stranger as he gazed upon hester

Answer 1

First of all, you need to know what a simile is. It refers to a figure of speech in which the author compares two or more things using words such as like or as. I managed to find two such similes in the part where the stranger sees Hester.
1.  It was carelessly, at first, like a man chiefly accustomed to look inward, and to whom external matters are of little value and import, unless they bear relation to something within his mind. - this is the moment he saw Hester
2. A writhing horror twisted itself across his features, like a snake gliding swiftly over them, and making one little pause, with all its wreathed intervolutions in open sight. - this is the moment when he recognized her; personally, I'd choose the second simile because it is more memorable