You’ve probably heard this writing metaphor before when it comes to characters: what is depicted on the page is just the tip of the iceberg, hinting at something more vast and complex beneath the surface. The vast structure beneath the surface of what you see depicted in a scene is the backstory of that character: what came before to make them who they are in the present. Even if those buried traits do not rise to manifest themselves directly in a scene of your novel, they still serve as an indirect influence in subtler ways.
I like to imagine my character alone in their bedroom. Maybe this room is a small studio apartment, or one of many in a grand mansion. Perhaps your character is a drifter staying in motel rooms or sleeping on strangers’ couches. Whatever the scenario, how they interact with that room will tell you a lot about them. Are they extremely tidy? Do they carefully fold each piece of clothing and stack it in the same place every night? Do they feel near panic at the slightest sight of dust and must clean it immediately? Why? What influences them to be this way? Did a family member from their past exhibit this same behavior? Does your main character still hear this family member’s demanding voice echoing in their mind? If the room is tidy or messy it reveals a lot about the character’s personality and backstory. Explore it.
What else can you describe about his/her private room? Does abstract art hang on the walls? If so, what does that tell you about your character’s way of thinking? Perhaps instead they like to display pictures of family. This tells you family is special to them. Why? Is their a particular family member they value most? All kinds of character revealing pathways to explore in the art and decorations throughout the room.
There’s one important question I always ask myself when exploring a character’s private room. This question really penetrates the heart of them, the juicy center: what secrets do they conceal in their room? It may be an object hidden in the closet or under the bed. What does that object mean to them? Why is it kept hidden? This question can lead to some fascinating answers about your character, and sometimes the answer is the course of a plot, which happened to me while writing a short story called “The Butterfly Girl” (unpublished). I discovered that a hat belonging to her father was very special to her, because it triggered precious memories to her mind about fishing with her deceased father when she was a child. Later she uses the hat as part of a conjuration ritual in an effort to contact her deceased father’s spirit.
The secret doesn’t have to be an object. It could also be an activity they practice alone that nobody knows about. Either way, exploring your character’s bedroom is an invaluable tool for character development and backstory.
Happy exploring!
A Writer's Journey 
Yes! I do this as well. It’s a great exercise in evaluating personalities. The MC in my first book has rustic looking quarters because he loves nature. The MC in my second book has only the basic necessities in his room and is fastidiously clean. He’s super-focused on his responsibilities and doesn’t care for distractions. The MC in my third book is similar to the one in my second, though he’s not quite as intense and not as strict.
Cool! You make thorough use of this method.