Narrative
Many games tell a story. Some stories feel like an integral part of the game, while others feel added on only to facilitate gameplay or a fun mechanic. In any case, the level you build will communicate a story, that is, the engaged player will unconsciously read the environment as they explore (its characters, architecture, and props) for meaningful elements relating to the overarching story. For this reason, ask yourself important questions about the objects that you, as a designer, add to a level, from tables and chairs to doors and windows: why put an object here? What does that reveal? How does it complement the story, if at all? For example, bloody footprints found on the floor speak on many levels to the player. They express harm, murder, and intrigue about an event in the past. They encourage exploration right now: where do the footprints lead, and why? Props such as this are important for generating interest and excitement. However, even a lonely, mundane object such as a chair can take on significance, for example, when positioned alone at the center of a large and creepy room. Who sat there? When and why? It's the curious juxtaposition and relationships between objects that make them interesting and meaningful, and so, it's important not to see the objects as filler, as simply things thrown in to use up space and reduce emptiness. Emptiness, in that negative sense, arises not so much from a lack of props and objects, but from a lack of significant objects, objects that make a difference and carry meaning. Therefore, add objects and design levels with consideration for purpose and narration; think about how objects relate to the story, which characters would have used them, and what their location now says about the recent events.