Emily Bronte's Wuthering Heights is a perfect example, where the house itself becomes a character in the book. Ms. Bronte flawlessly wove personality into the surroundings by using melancholy and unnerving descriptions so that the house and setting emanate actual emotions: anger, hatred and jealousy.
Here's a quote taken from an online essay: “Wuthering Heights ... suffers from a kind of malnutrition: its thorns have become barren, its firs stunted, everything seems to crave for the ‘alms of the sun’ that sustain life.”
In one of my historical love stories, there's a pair of 16th century Italian shoes which harbors a gypsy curse and has an amusing yet creepy tendency to move about without a wearer. The heroine is drawn to the shoes, almost to the point of obsession. They hold a mystical power over her, even without her realizing they also hold the secret of her lost past.
Another example is my gothic literary love story, where a flower which embodies a man's spirit becomes an active participant in the intensely emotional relationship between the ghost and the flower's keeper, a young deaf woman.
Even in my SPLINTERED MS, worn-out and mutilated toys play too big of a role to be considered mere objects.
Anytime an "inanimate thing" serves as a game player or mirrors the characters and their arcs, it evolves to more than just a prop. It takes an active role in the plot, a role that without which, the story wouldn't survive. Thus it becomes -- for lack of a better description -- a character. Within the confines of the story, it develops a soul.
Looking back on your stories or WIPs, can you find any objects that could be considered pivotal characters?