What can I write for a thesis statement for an essay about how the settings influence a story's characters? I don't know how to take a stand on this type of essay.

Make a list of half a dozen or so characters you've read about. What is the setting for each character? Can you imagine them in a different time or place?

For instance, I'm reading a mystery novel set in the Pacific Northwest in the present. The three main characters tend to be loners and are very familiar with the outdoors. One lives in a primitive cabin and eats rabbits and other game. They've adapted to their environment.

On the other hand, one of my favorite series of mysteries features a medieval monk in medieval England. If he lived in a different time and place, he would have been a different person.

So, could I say that characters and their settings give the story its unique plot?

You need to be more specific.

Have you listed the characters that you know?

Yes, I listed them.

Did you list their settings?

Can you imagine any of the characters living in a different time and place?

Yes, and it seems like the characters are best suited for their given settings.

Then your thesis statement could be

Settings influence the characters of stories.