In my story, there are two characters: a schoolgirl and a strange person with a creepy mask who gives her a tour in an art museum at night. Well, actually there is a third character; the boyfriend of the girl, but he only answers the phone at the beginning to tell her that he will come later (but never does), and she talks about him all throughout the novel.
Will this short novel (approximately 200 pages) be boring to my readers because it only has two characters, and because the whole novel is set in that art museum? Will it be better if there are more? Or should I just stick with the plot without caring about that?