Citizens only appear when there are Businesses with available goods for sale somewhere in your city. If all businesses are unavailable due to either being ready for collection, or needing supplies, Citizens will begin to make their way back to their homes.
The happier your citizens are, the more likely that more citizens will move in. For happiness levels, see the City Population page.
Special Types of CitizensEdit
Special Citizens will appear on the streets of your city only when specific buildings are built. These Citizens behave in much the same way as regular ones do, just with different model types than previously seen. If the buildings that triggered the new Citizen are removed, the Special Citizens will disappear as well.
See tourists, for people with 'Green Dollar' signs or 'Briefcases' over their heads.