In "The Monkey's Paw," how does the author use characterization?
Characterisation refers to how the author presents, describes and uses the characters in their work to advance the theme or message of the story. So really, in a sense, what you need to do to answer your question is to think about how the author presents the various characters and uses them to enhance the horror of this tale. For me, one of the characters who really stands out, is Major Morris, who gives the White family the poisoned chalice of the monkey's paw.
What is key to realise is how he is presented. He is described as a "tall, burly man, beady of eye and rubicund of visage." The fact that he is a Sergeant Major and physically strong is made evident, an impression heightened by his experience of active service in India:
...spoke of wild scenes and doughty deeds; of wars and plagues and strange peoples.
Having established the strength and power of this character, the author then shows the true terror of the monkey's paw by describing its effect on the Major. Note how in response to being asked if he has had three wishes, he responds, "I have," and we are told something that we would not expect from such a man: "his blotchy face whitened."
The author therefore uses the characterisation of the Major to foreshadow the tragedy that is to come in the rest of the story. For such a straightforward, physically powerful man to respond so strongly to a "fakir's trick" clearly points towards the truth in the monkey's paw - a truth that the White family find out for themselves all too soon.