If no one buys a ticket, do they still show the movie?

 Sometimes yes and sometimes no.  My opinion is sometimes yes and sometimes no.

 At Cinemark the audio continues to play but the screen will go dark if the system shows zero tickets were purchased.  

 AMC Sony projectors will power down after 30 minutes, while the other branded projectors will not.  

 Independent small theaters rarely waste the bulb hours on an empty theatre.  Exceptions typically being contractual agreements with distributors.