How can I remove a crowd of people from a scene while keeping one person in the shot I'm trying to film in a city with one person being the focus point, and the goal is to make the city look deserted.  If the city is always busy and full of people, is there a way for me to remove the people in After Effects or Premiere Pro?  I would think that one could just make a composite shot of only the parts of the image that stay constant, but I have no idea how to even approach that.  How can I do this?
Is the camera moving? If it's a still shot you could make a clean plate of the empty city in Photoshop and use a green screen for the actor. If you don't want the shot locked-off you can cheat add some movement to the frame in post, but big changes in perspective would require recreating the city in 3D. 



Interestingly one of the earliest photographs of a human being was by accident — owing to the extremely long exposures required by the early emulsions, anything moving would blur out. In a photo by Louis Daguerre there was a guy standing having his shoes shined and he appears to be the only person in Paris. You could use the long exposure technique to get your background.