interiors

Model railroaders are a weird bunch. We like to see cluttered structures and tons of stuff just lying around inside buildings, at least we do if the clutter and stuff is within a scale model structure. When you build a structure with open doors, you've got to add some kind of detail to the structure's interior. Think about your goals in modeling the structure and what would realistically be inside that structure. If you're modeling an empty building, there should be a "for sale/lease" sign or contractors' trucks and equipment outside to explain why the building is vacant. If the building is occupied, fill it with appropriate details, like in this scene of a small garage on a narrow gauge logging railroad.
 

Adding interiors to your foreground structures can not only get you extra points on NMRA contest judging, but their presence on your layout makes the viewer believe that your background buildings are also as detailed, even when they aren't. The model pictured here was entered in the structures category at the Seattle convention.