When you are trying to establish a geography for your story, or when you are just trying to keep a “real world” feel, you are going to need other people around other than the main focus of the story. It would feel really awkward following the main couple through a supermarket where they were the only patron’s, or working out in a gym that is completely empty. The best, and cheapest, way to do this is to start gathering your friends together and get them conglomerated in your scene.